The issue with the phone number. It’s probably one of the most common criticisms you hear about Signal. And that’s saying something. Otherwise, with its focus on security and privacy while maintaining user-friendliness, Signal offers little cause for criticism (we have fact-checked common criticisms and myths about Signal in a separate blog post). But why do you need a phone number to register at Signal? Here we’ll clarify this question and show that there are good reasons for this.

How does the registration process at Signal work?

First, the facts. To register with Signal you need a phone number. This can be either a mobile or landline number. This number is then confirmed during registration with a verification code and linked to your Signal account. You can find details about registering with Signal on this official support page.

But what is the reason for the decision to require a phone number during registration? Why can’t you simply sign up at Signal using an email address or any username?

This is the answer that Signal’s president Meredith Whittaker gave to exactly this question in 2022:

Phone numbers allow people to ‘import’ their social graph into Signal and, more importantly, take it with them if they decide to stop using Signal. The network people bring with them via phone numbers helps people quickly connect with their friends and colleagues. And, of course, that network—the network of people you care about, want to collaborate with, and want to talk to—is at the heart of a functioning messaging service.

Signal without a phone number? – Empty and unsuccessful

What would a world look like where Signal wouldn’t require a phone number for registration? After installing Signal, you simply wouldn’t know which of your friends can be contacted via Signal. Of course you could call all of your friends and ask if they’re on Signal and what their username is. But that’s not really practical for a messenger. And Signal believes that only a practical messenger is a useful and secure messenger. After all, what’s the value of a (theoretically) secure messenger that no one uses? Exactly. That’s why Signal has opted for a compromise.

By registering with your phone number, you can get started right away. Everyone who installs Signal can see which of the existing contacts are already on Signal and can immediately start writing secure messages. This explains the great success of Signal. Because unlike many other encrypted messaging apps I can get started right away and my friends are notified that I’m now on Signal as well.

Additionally – as Meredith Whittaker mentions in the quote – this allows you to remain in “ownership” of your social network as much as possible. Assuming Facebook shuts down tomorrow (one can still dream) you’ll lose all your contacts there in an instant. On the other hand if you build a social network based on phone numbers it’s independent of the network provider. So if Signal discontinues its services tomorrow I can at least continue to maintain the contacts I’ve made by phone number and won’t lose them completely.

Also helps fighting spam

As Signal President Meredith Whittaker confirmed there’s another reason why Signal requires a phone number for registration: combating spam.

Of course it’s relatively easy to obtain new phone numbers and misuse them for spam purposes in some countries. However, this isn’t as easy or as inexpensive as email addresses for example. Therefore, the phone number requirement also makes it easier for Signal to combat spam and thus further increase Signal’s user-friendliness.

Signal hides phone number by default

While you need a phone number to register in the beginning it doesn’t mean that your phone number is exposed to all other Signal contacts. Since February 2024 Signal hides your phone number from your Signal contacts by default. If you use Signal, your phone number will no longer be visible to everyone you chat with (you can change that setting in the settings of Signal). There is only one excpetion: People who have your number saved in their phone’s contacts will still see your phone number since they already know it.

So in the end the fact that you need to register with a phone number justs leads to one small piece of data: Signal itself knows that this phone number is registered at Signal. But since everything is end-to-end encrypted Signal doesn’t know anything else. Your messages, media, profile, your group memberships, who you talk to all this information is encrypted and hidden from Signal. This is even true for your contact list.

Signal doesn’t know your contacts

It seems to be logical: In order to determine which of my contacts are already on Signal, Signal must know my address book, right? If that would be true that would be a serious privacy problem.

Fortunately that is not the case for SIgnal. As Signal states its support pages:

Signal developed a private contact discovery process that enables Signal clients to efficiently and scalably determine whether the contacts in their address book are Signal users without revealing the contacts in their address book to the Signal service. Once your phone knows which of your contacts is a Signal user, it can optionally notify you when a new contact has started using Signal.

Signal has made a significant technical effort to address this question in order to be user-friendly while protecting your data. Signal founder Moxie Marlinspike has explained the technical details and general considerations in two detailed blog posts. If you’re interested in more detail, you can find them here and here.

Summary

These findings can be summarized as follows: The fact that a phone number is required in order to register at Signal brings large benefits and very little drawbacks.

Benefits

• You get the benefits of being able to chat with your existing phone contacts immediately and without the hazzle of looking them up manually.
• You own your social contacts and are not depending on a commercial service with the danger of loosing all your contacts if the service shuts down.
• It’s harder to fake profiles and easier for you to confirm that you are actually talking to the people you think you’re talking to
• It’s harder and more costly for spammers to register.

Drawback

• Signal knows that this phone number is registered at Signal (and the date it was registered and last connected to the service). But that’s it. Signal doesn’t know your name, your messages, your calls, your profile, your metadata, your groups – not even your contacts.

So in the end everybody has to decide which of these things are more important to them. As this article has tried to show there are certainly many good reasons to require a phone number at registration and it surely is one reason why Signal is the most popular secure and private messenger worldwide.