I have a Huawei E353 modem which I'm using with a Raspberry Pi.
When I plug it in, I get three TTYs - ttyUSB0, ttyUSB1 and ttyUSB2.
But sometimes the modem interface (where you can issue Hayes-type AT commands) is on ttyUSB0 and sometimes it's on ttyUSB1.
I can't spot any common factor that makes the difference. And I'm not certain, but it does seem that it always does the same thing on the same Pi (or maybe the same dongle - I've got several of each and haven't had a chance to try different combinations too much).
Can anyone suggest what influences the tty numbering?