I recently bought a modem that supports CSD (Circuit Switched Data) calls. I put SIM card in it and I am able to make calls via AT commands. But these are always VOICE calls. I have another modem and I need to connect to it via CSD.
I know it is possible because our colleagues are using it this way to access some network.
They are using windows dial-up connection, but I guess that is just something that in the background calls the AT commands.
Nevertheless the dial-up is still giving me "Error 678: Remote computer did not respond". I have tried many settings, but still the same error. I even tried different computer and still got the same error.
So I found out that when I call CSD, then on the receiving side (if AT+CRC=1) it should display +CRING: REL ASYNC. So I setup another modem and called it and it showed +CRING: VOICE.
So how do I convince my modem to call in the REL ASYNC mode? I already tried:
AT+IPR=115200 or AT+IPR=9600
AT+FCLASS=0 (this is default though)
AT+CSNS=4
AT+CBST=7,0,0 or AT+CBST=71,0,1
AT^SXRAT=0 (this changes the network to GSM)
But still when I do ATDNUMBERIWANTTOCALL; the receiving side shows +CRING: VOICE.
Also my mobile provider ensured me, that the CSD technology is still supported in my country and that I do not need any kind of special SIM card.
In the end the number could be called normally like it is written in the question, but it only worked in Germany, not abroad...