how to detect usable /dev/ttyUSB* port after switiching to modem mode
 

Hi all. I want to find out correct /dev/ttyUSB* port which can be used to be dialled by pppd. I actually tried sending some AT commands but i'm getting response for /dev/ttyUSB1, /dev/ttyUSB3.

There is probably no way to reliably predict what device node will be associated with USB serial interfaces. As devices get plugged in and unplugged, the assignments will change. The only recourse I know of is trial and error. You may be able to contrive a udev rule to make a consistent mapping based on a serial number that I think most/all USB serial devices expose. I can't provide any details on how to accomplish that.
--- rod.