A problem with number portability is that users can’t know which operator their friends are at. So fixed rate plans have a big issue as you don’t know if your friend is within your fixed rate plan or not (often fixed rate only covers on-net). So an IM interface could also include this information so that users know who they can call for free.

This isn’t really a new idea, as it’s been part of the IMPS specification. But I don’t think it has actually been implemented…