I have three separate SIMs, er, three separate phone numbers (I only use one for voice calls, though, and ... Though this isn't the main point of the article,

I have three separate SIMs, er, three separate phone numbers (I only use one for voice calls, though, and ... Though this isn't the main point of the article, it resonated with me as I've always wished for phones that can take multiple SIMs at once. This would be so useful: business/personal numbers in one phone, or when abroad, simultaneously using a local and a home (roaming) SIM, etc. Clearly, the network operators have a vested interest in discouraging you from using anything but their service, but is there a technical constraint in GSM/UMTS that prevents one phone (with 1 antenna) from logging onto multiple networks at once?