In this article, we explain how it works, what you need to know if you receive a call from this prefix, and the exact way to check whether a number belongs to Endesa so you can avoid potential scams.

What is the 400 prefix and what is it used for?

By definition, the 400 prefix is the new nine-digit national number range allocated exclusively to outbound sales and marketing calls made by businesses.

This measure has been approved by the Spanish Government to increase telephone transparency, protect consumer rights and provide a clear way to recognise promotional calls before answering. One of the most important technical features of this new prefix is that it is one-way only: numbers starting with 400 are configured solely to make outbound calls, not to receive inbound ones. This design is intended to prevent unexpected phone scams that often occurred when people returned missed calls to unknown numbers.

The 400 prefix is not exclusive to Endesa

Any company operating in Spain can use these lines for its marketing campaigns. Seeing the 400 prefix on your mobile screen immediately tells you that the call is commercial in nature, but it does not identify the specific company or guarantee that the caller is genuine.

Key facts about the 400 prefix

• Commercial purpose: It is a 9-digit national number range dedicated to selling and promoting products or services.
• One-way only: It can only be used to receive incoming calls; the network blocks any attempts to call these numbers back.
• Used across the market: It is used by different companies across the market and does not belong to any single business.
• Clear intent: It lets you know straight away that the caller is contacting you to offer you something.

When did the 400 prefix come into force in Spain?

Mandatory use of the 400 prefix for sales calls came into force in Spain on 17 October 2026. Since that date, companies have been prohibited from making sales or customer acquisition calls using standard telephone numbers. This measure is backed by an official regulatory framework comprising the Customer Service Act (SAC) and Ministerial Order TDF/149/2025 under the Plan Against Telephone and SMS Scams.

Compliance with these measures is overseen by official bodies including the Ministry for Digital Transformation and the Civil Service, the National Markets and Competition Commission (CNMC) and the Telecommunications User Support Office (OAUT).

Which calls are required to use the 400 prefix?

Companies may contact consumers for a variety of reasons. To ensure clarity and security, the regulations draw clear boundaries between calls depending on their purpose:

Sales calls offering products and services

The 400 prefix is mandatory whenever the purpose of the call is promotion or sales. This category includes:

• Offers and promotions: Sharing discounts, limited-time promotions or new tariffs.
• Customer acquisition: Outbound prospecting calls to potential new customers.
• Sign-up campaigns: Proposals aimed at taking out new contracts or switching contract types.

Customer service calls

Calls that form part of after-sales support or day-to-day contract management do not use the 400 prefix. These interactions must be carried out using standard geographic numbers, freephone ranges (800 and 900 lines) or authorised shortcodes. These include:

• Contract or billing queries: Clarifications regarding bills, recorded usage or current contract terms.
• Fault and incident management: Updates on technical faults, operational support or ongoing complaints.
• After-sales support and assistance: Ongoing advice regarding the services you have signed up for.

Calls relating to a previous request or application

If you have expressly requested that a company contact you (for example, by asking for information about a tariff on its website), the call is classified according to the purpose of the contact. In other words, if the aim is to present you with a sales proposal to sign up, the call will be made from a 400 number. Under the regulations, companies are not allowed to make additional sales pitches during a customer service call, unless they are directly linked to resolving your query and involve a clear improvement in the terms or price of your contract.

How can you tell if a call from a 400 number is genuine?

All you know when you receive a call with a 400 prefix is that it has a commercial purpose. Nothing more. This prefix does not identify which company is behind the call. It is therefore advisable to remain cautious and always verify the caller's identity – especially before sharing any personal or bank details. If you have any doubts about whether the call is genuine, remember that you can hang up at any time.

Warning signs of a potential phone scam

To protect yourself against potential fraud or impersonation attempts, look out for these common red flags:

• Urgency or pressure to sign up: Pushing you to make an immediate decision or switch contracts during the call, without giving you time to think it over.
• Threats of supply disconnection or penalties: Alarmist claims warning that your supply will be cut off immediately unless you accept the offer on the spot.
• Suspicious requests for sensitive data: Unsolicited requests for bank account numbers, passwords or private personal details.
• Offers that cannot be verified on official channels: Promotions whose details do not appear on the company's official website or channels.
• Callers claiming to represent a company without proof: Callers who claim to be phoning on behalf of a brand, but provide no way to verify that the call is genuine.

How to check if a call is genuinely from Endesa

For us, our customers' security and transparency in every communication are top priorities. That is why we have already added all the 400 sales numbers from which we may contact you to our official verification tool. So whenever you receive a call from Endesa on a 400 number, you can check whether it is genuine.

How to use the tool step by step

1. Note down the number: Save the nine digits shown on your phone screen.
2. Go to the Endesa number checker: Visit the company's official website to use the tool.
3. Check the result: Enter the phone number in the search box and the system will tell you immediately whether the number belongs to an authorised Endesa sales channel.

Important: If the number does not appear as registered, we recommend that you do not share any information whatsoever.

Can you call back a number starting with 400?

400 numbers cannot receive incoming calls. This is an additional safeguard, as some scams rely on the 'missed call' trick, charging high rates when you return the call.

If you want to contact the company that called you from a 400 prefix, the safest way is to get in touch through its official channels, which you can usually find on the company's website.

In Endesa's case, you can reach us on these numbers:

• Freephone customer service line: 800 76 09 09
• Tariff advice line: 919 101 143

You can also find more ways to contact us here.

What to do if you receive a suspicious call claiming to be from Endesa

Hang up, check and protect are the three golden rules to follow when you suspect that someone claiming to call from Endesa is not genuine. By doing so, as well as staying safe, you help us maintain a transparent and constructive relationship with our customers.

1. Hang up if you are in doubt. Prevention is the best defence when something feels suspicious. If it really was Endesa calling you, it is because we have your consent – meaning we will call back in the future. This gives you time to apply the next rule: check.
2. Check whether the telephone number matches Endesa's official numbers. If you are unsure, you can contact us directly to ask.
3. Always protect your bank and personal details. Never share them unless you are completely certain that the call is legitimate.

Frequently asked questions about the 400 prefix

What does it mean if a number starting with 400 calls me?

It is a sales call. Since October 2026, all sales and marketing calls in Spain must be identified using the 400 prefix.

Where is the 400 area code from?

The 400 prefix does not correspond to any geographical location. It is a nationwide identifier used across Spain for calls with a commercial purpose.

Does the 400 prefix belong to Endesa?

No. Endesa has its own numbers starting with 400 for marketing communications, but every other company's sales numbers will also start with 400.

Is it safe to answer a call starting with 400?

The 400 prefix is a security measure. The Government's aim is to ensure that consumers can clearly identify incoming sales calls, allowing us to decide whether or not to answer them.

Are calls from the 400 prefix spam?

Not necessarily. Calls starting with the 400 prefix are not automatically scams or illegal spam; rather, they officially identify legitimate marketing calls made by businesses.

Is it mandatory for sales calls to start with 400?

Yes. From 17 October 2026 onwards, all numbers used to make outbound sales calls must carry the 400 prefix.

Can Endesa call me from a 400 number?

Yes, when the purpose of the call is to offer you a deal, product or service. These numbers are kept separate from calls relating to the day-to-day management of your account with us.

How can I check whether it was really Endesa that called me?

On Endesa's contact page, you can check the number that called you against our official telephone numbers. If you have any doubts, you can ask us directly by calling those numbers or via live chat.

Can I call back a 400 number?

No. Numbers starting with 400 cannot receive incoming calls.

How can I block sales calls?

You can block the number on your mobile phone or mark it as 'spam' or 'unwanted'. You can also tell the caller that you do not wish to be contacted again and ask them to remove your details from the company's database. And if you do not want to receive any marketing calls at all, you can register with the Robinson List.

What is the difference between 400 numbers and 900 numbers?

400 numbers are used for incoming calls to you and identify that the call has a commercial purpose. 900 numbers are freephone numbers that you can call yourself.

Why is a company calling me if I haven't given them my consent?

This usually happens because you gave consent to a related company and agreed that they could share your details with third parties. To withdraw that consent, you can tell the company that calls you directly.

What should I do if someone impersonates Endesa?

Hang up as soon as you suspect anything, avoid giving out any bank or personal details during the call and, if it is a clear scam attempt, report it to both Endesa and the authorities.