After getting two modems (dev boards) Siemens TC35 and SIM900A I cannot get them to send SMS. From the research it seems that somehow SIM cards will not register to network. I bought three SIM cards from different networks currently operating in my location.
Commands to GSM module:
AT OK AT+CREG? +CREG: 1,0 OK
All SIM cards have their pin security check disabled. All cards have been inserted into phone, tested for a call and tested to send SMS to another phone. Also all of them all full of credit.
Just tested cards with GM862 module with same commands and all works fine (sending SMS works withut any issue)
For reason unknown to man kind, and all the frustration I've decided to run TC35 without SIM card, and add SIM card in the process of running.Running
AT+CFUN=1 , and then trying to sens SMS proved to work. I've then unplugged TC35 from power and then connected it back and now it works normal. Only god of electronics and mights of circuits know what the hell is going on there.
So for know only SIM900 still doesn't work.