Last week I was in a close-out store and found a bunch of Net10 prepaid Nokia 1600s for $20 each. At first I thought I'd found a good source of cheap handsets for testing. I got one home and even though I provisioned it in my OpenBTS system, and even though it registered and showed service, it refused to place a call without any minutes in its "tank".
Here's what I did find, which may be of interest. First, the SIM was generic-looking, no corporate logo, just the letters "SIM" printed on it. Second, when the phone tried to register, the IMSI was from AT&T: 310410226242003. Third, the phone rejected other SIMs, including other AT&T SIMs. The handset appears to be keyed to a specific SIM, so to get this handset to act like a normal phone I'd need to get it rebranded, not just unlocked. Fourth, menus in the phone showed the IMSI, the IMEI, the phone number and a "random number". That was unusual, since a handset normally does not know its own phone number. I am also eager to see if that "random number" is really Ki.
So I won't be buying a big pile of Nokia 1600s at Big Lots, but I'm keeping this one phone because it will be a great opportunity to see how prepaid phones interact with the network. Hopefully, in a couple of weeks I'll have a chance to play with that, unless some other OpenBTS developer out there beats me to that. (Hint, hint...)