I feel like that issue could be solved by creating a random unique say 16 digit UID that users could then add as a phone number for contacts. As long as it does not map to a real routable phone number a phone dialer won't have issues if you call it by accident. Smarter dialers like that in calyxos could recognize it for signal calls if it has a unique prefix.

They could even sell existing users UIDs so they could share their contact info without sharing their phone number if they so desired, e.g. via a website. If they charged some nominal fee users could reset their UID. Maybe with escalating prices if done in short succession to make revenue and discourage abuse.