IMHO, I haven't heard anyone telling/writing down their actual phone number for quite a long time, what I see happening a lot it's stuff like "you want my phone number? sure, let me give you a short call so that you can save it in your's phone's address book" (with the option "you want my Vodafone or Orange number"? I haven't actually never seen anyone changing their SIM cards, people just prefer to own 2 or more phones if it best suits their needs. Phones are getting damn cheep, at least in Eastern Europe).
What I would expect is a protocol where both just should be able to connect their phones on short distance (bluetooth, whatever) and acknowledge an "exchange phone number with <person>?" dialog ...
Handy, but the problem is that you still need a number for SMS, and Bluetooth is damn slow, particularly if you're not paired with the other device.
P H O N E C E P T I O N