I'm using a Huawei GMS/UMTS USB modem with Gnokii and Gammu.
The modem was in the lower port on Pi. With
dmesg | grep tty it would show up on
/dev/ttyUSB1. (2 USB devices on 1 stick is normal, in this case).
I configured Gnokii and Gammu accordingly, however on ttyUSB0 I could only send messages. Both sending and receiving would only work over ttyUSB1.
I then moved the stick to the upper USB socket (because otherwise my WiFi-stick wouldn't fit next to it). Everything kept working fine. Also after several reboots, cold and warm.
I then needed to expand the file system (Yeah, I know I should have done that earlier). After that, the modem was on ttyUSB0 and ttyUSB*2*. Send/receive worked on the latter. This took me some time to find out.
Update: 2 days later, ttyUSB2 has disappeared. The working modem now manifests itself on ttyUSB0 (was ttyUSB1).
Any idea why this happened? Any links to relevant debian or linux docs, perhaps? I'd be much obliged..