I'm have board with sim900 connected to pic. Modem have 4V supply, pic have 3.3V.
SIM900 use TTL 2.8V true uart, not inverted. I put just 2 resistor in series with comm lines.
If you connect modem to rs232 side of max232, probably you frayed modem port.
1. Connect modem and usb board, and check that.
2. I tell you to try to connect your usb board to PIC instead of modem board. So you can check that communication is ok on PIC side...
You didn't try that...
So try to send response from PC to PIC. Only in HSERIN put longer timeout, eg 5s..
3.You said in post 10 you menage to get signal level. So your hardware setup was ok.