Phone numbers are unique values when you need to call a phone or send an SMS, which is what this software does. If you were trying to do your taxes, sure...but in this case a phone number is exactly enough.

Lots of folks still use land lines.

They probably aren't trying to use TextSecure with their landlines though.

You should never assume that any piece of information is not enough to uniquely identify an individual.

You are thinking from the point of view of a database administrator, whose job it is to make it easy to uniquely identify an entity.

In security it's about making it as hard as possible to deduce even the slightest of information about a person, increasing the k-anonymity as they say.

For all intents and purposes, that a phone number resolves to only two people, and they're also conveniently located geographically near to each other, and are probably very distinct in a whole range of other things, makes it the perfect key for any security adversary.

You make a good point, my job is to make it simple for machines to differentiate between two people, a task a human could do easily.