According to Mary Jo Foley, Microsoft are currently making a new platform — code-named Echoes — designed to be implemented by (mobile/cell) phone companies, aka carriers, so that not only can Microsoft try and stay on top of the game by encouraging more of these companies to deploy Windows Live mobile services to their customers, and do it more easily, but also so that the customers (us!) get these useful features:

With part 1 of Echoes (scheduled for summer 2008, approx. July):

• Synced Windows Live Contacts on your phone — Your contacts from Messenger appear in your phone's address book. Among other things this means that you don't have to know a contact's phone number to call them as each contact will have their mobile number 'assigned' to them (hence Bill saying "We can get rid of phone numbers" in another one of his good old visions of the future). It seems that the Echoes platform will push new contacts and contact updates to your phone without it needing special client software.

• Simultaneous ringing — Presumably similar to how simultaneous ringing (or dual forking) works with Office Communications Server and how MPOP [multiple points of presence] works in Windows Live Messenger 9 (currently in private beta). It will probably mean that when you ring a contact from your phone it will call their phone and PC and they can choose which one to answer, and vice-versa so that you can call a contact's phone and PC from your PC.

• IM to SMS continuity — A bit like the current mobile messaging integrated into Messenger. Allows you to send a text message to a mobile contact (who will always appear 'online' or similar if they don't choose otherwise?) from within Messenger and their SMS reply returns to you as an IM.

And with part 2 ...

• Who knows? Integration with landline phones? Synchronisation with your schedule/calendar and mail? Your phone's voicemail in Messenger? It's all unified communications!

So is this why we're still waiting for another beta version of Messenger 9? Foley also says:

The full extent of Echoes should apparently be seen in the next year or two.

Source: All about Microsoft: Microsoft's grand plan to eliminate phone numbers