Do you have to call from a 400 number? What changes in your sales calls

This article is for informational purposes and does not constitute legal advice.

Your phone rings mid-morning. On the screen, a +34 400 followed by six more digits. You don't need to pick up to know what it's about: someone wants to sell you something.

Now put yourself on the other side. If you sell by phone, your calls arrive with that label attached. And any lead who couldn't pick up and tries to call you back finds no one on the other end.

In this guide you'll see, in five minutes, whether the regulation applies to you, what calls count as commercial, what changes if an AI agent is calling, and how not to lose someone who wants to return your call.

What is the 400 number and since when has it been mandatory?

The 400 number is the mandatory numbering in Spain for commercial calls as of October 17, 2026. It only makes outbound calls: it does not receive calls, nor is it used for customer service. Law 10/2025 requires it for basic service companies and large companies selling to consumers.

It works like a return address on an envelope: the person knows who is calling and why before answering. If someone dials that number to call you back, they won't reach you. It is also not used for support or complaints.

Where the obligation comes from

Law 10/2025 on customer service requires separating customer support and commercial calls with distinct numbering.

The Resolution of April 14, 2026 reserved the 400 prefix for commercial calls and granted a six-month grace period, which ended on October 17.

Which companies are required to comply

The obligation does not apply to every company that places calls. It affects two groups:

• Basic service companies: water, gas, electricity, passenger transport, postal services, telecommunications, and financial services.

• Companies that sell primarily to consumers and that exceeded, either alone or as a group, any of these thresholds in the previous year: 250 employees, 50 million euros in turnover, or a balance sheet total of 43 million euros.

If you are not obligated, rules still apply to you

If you are not in either group, the law does not require you to use a 400 number. But Article 66 of the General Telecommunications Law and the AEPD's Circular 1/2023 apply to anyone calling to sell.

And vice versa: calling from a 400 number does not replace consent. The number identifies the call; it does not make it legal.

What calls count as commercial?

The purpose dictates the rule, not the campaign name. Calling it "follow-up," "qualification," or "reminder" changes nothing if what you are after is for the other person to buy, sign, or agree to a meeting.

The tricky case: mixed calls

An appointment confirmation that ends with "by the way, we have a promotion this month" is already commercial.

If your team does both, separate them into two calls and two numbers. And calling a company instead of an individual does not change the nature of the call.

What changes if the call is made by an AI agent?

Having an AI call doesn't make the call any more or less commercial. An agent calling leads to book a demo needs a 400, just like a human sales rep. One confirming appointments or handling reception does not.

A voice agent is, in practice, a teammate talking on the phone following a script (the prompt) that you write for it. The rules of the call are the same as for a human.

Two obligations that carry more weight than the number

• Disclosing that an AI is speaking. Article 50 of the European AI Act may require you to notify the other party that an AI is speaking. We explain this in our guide on the AI Act and voice agents.

• Having prior consent. Article 66.1.a of the General Telecommunications Law may require it for commercial calls made without human intervention. This is the case for an agent running the campaign on its own.

Changing the number is the easy part. The hard part is being able to prove why you have permission to call each contact on the list.

🎧 Want to hear how it sounds? Listen to a sample call of a Diga agent introducing itself as an AI, qualifying the lead, and leaving a contact number. Listen to the demo

No one can call you back: how not to lose the contact

This is the most frequently overlooked detail. Someone who couldn't pick up and calls you back shortly after dials the 400 and reaches no one. They were probably the most interested contact on the entire list.

The solution is to give them another path before hanging up:

• A local geographic number that does receive calls.

• A contact email address.

• A link to book an appointment.

With a voice agent, simply add it to its instructions. Here's how the end of the call could sound:

Agent: Perfect, I'll send you the summary by email. One thing: you can't call this number back, so if any questions come up, call us at 910 000 000.

Customer: Okay, can you repeat that?

Agent: 910 000 000. I'll also leave it in the email for you.

If that local number has an inbound agent (one that answers the calls you receive), the return call is also handled by AI, at any time of day.

How to configure a 400 number in Diga

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1. Buy the number. Under Phone numbers, click "Buy number" and select Commercial (400) in the prefix selector. It costs the same as any other number and doesn't require a provincial utility bill: if your project is already verified, it activates upon payment.

2. Use it as the caller ID for your campaigns. Select it in the "Make call" action of a workflow or, if you initiate calls from your own software, specify it as the from_number in the API. Diga will not let you assign an inbound agent to it, because it will never receive calls.

3. Make a test call to your mobile phone and verify that the 400 appears on the screen.

You can find all the details in the 400 numbers documentation.

Nail the first few seconds

The 400 tells the customer you're going to sell them something before you even open your mouth. The first few seconds of the call matter more than ever. Spend more time on that greeting than on any other part of the prompt.