You can have it seems a text to any number to verify so even a dumb mobile phone would work.

Though I totally agree linking an IM account with a mobile number is bad. There's benefits too like discoverability and ease of use but I prefer something where I could be anonymous and have multiple accounts.

yes, signal requiring a phone number is the problem, and the only reason why I am not using it. In the last 10 years I have lived in 4 different countries, each time switching phone numbers. It is stupid to use such an unstable number as identity. My e-mail address has remained constant during the whole time. And I do like to have different addresses for different groups of people.