In fact, it is depended on the ability of your GSM module.
Here are 3 methods to try to solve your question, FYR.
1. to monitor the responses/result codes from the GSM module
also depends on E,Q,V,X commands
2. to monitor the RLSD/CD (Received Line Signal Detector/Carrier Detect)
also depends on '&C','&D' CD/DTR behavior commands
3. to request the call meter via GSM AT commands
+CAOC/+CACM for current/accumulated call meter
For more details, to read valued documents (07.07) from
ETSI or
3GPP.