Which is still a stable identifier that other people know you by, so you will likely keep it a long time and amass a trail of location data. Also its trivial to tie to the IMEI, so if you actually want to change nyms you have to buy a new phone as well.
Everything about the legacy phone system is a liability. Contact discovery is difficult, but tying into phone numbers should be optional and only for the duration of setting up a contact. Using phone numbers for long-lived identifiers is insane.