Hi!! I am Enrico and would like to say hello to all of you, PJSIP community!! First of all, thank you for your work in PJSIP. I am impressed by the capabilities of the library! So I am trying to build a SIP UA acting as a GSM gateway, more or less. In particular, my idea is to build an application connectin to a PBX, and allowing to use 3G/4G modems with suitable funcitonality to make/receive phone calls. I would need to be able to handle more than one modem at a time possibly. It seems PJSIP is the right library for the task, but I am very new to it and would need some guidance on how to proceed, and what functionality, and functions, I could use. the simple_pjsua.c example looks similar to what I need, except for the fact I don't need to interact with sound devices. I know pjsua_set_nno_nd_dev is what I need for this, but I am not able to understand the logic flow of things, and how to use them. the setup I have in mind is as follows: 1 - I would need to develope a port to read/write to a GSM modem serial device: it produces RAW PCM audio, which I can read with a command like: play -t raw -r 8k -e signed -b 16 where 2play" is from sox. Is there already a port for doing something like that? I tried to look at the wav_player / wav_writer, but don't yet know if this is what I need. 2 - I would need to react to SIP events and Modemmanager events, making and receiving calls accordingly. What tools do I need to acomplish this? For me, even understanding how PJProject's memory pools work seem a challenge. Can you help me? :) The project I am working is GPL V2+ code, so I plan to share it once it's working! My objective is to do my best, and understand how things work to use them properly. Thank you!! Enrico