I do not think it is a bad idea, but doesn't this at least canonically break invitations to lots as well? If they are not registered in your phone you would have sacrifice contact of both sorts, being it normal phone communication or travelling together. Personally, I am not sure I am willing to risk having a Sim I wanted to develop a relationship with disappear and then have to chase him/her down in god mode and transfer families. It would be a lot of work.
Theoretically, though, this would be the exception to the rule and stranger calls are annoying. I kind of feel there needs to be a "phone book" interaction on the computer where you can look up NPC numbers. Better yet, a phone interaction to block certain numbers. That way you can keep people in your phone and turn them on and off, if you will.
