The command is for SIM900. I don't know command for SIM300. Read SIM300 AT Command manual. For SIM900 if you give
AT+CLCC
you get
AT+CLCC
+CLCC: 1,0,3,0,0,"99xxxxxxxx",129
OK
as response.
You can see 3 in the response. The position corresponding to 3 in response have different value during dialing, after dialing and if call is active.
https://www.activexperts.com/serial-port-component/tutorials/gsmdial/
Edit: If you want to control device from any modile then give the below command to receiver GSM modem.
AT+CLIP=1
//Send once
AT+CLIP=1
//response
OK
//Incoming call is notified as below
RING
+CLIP: "xxxxxxxxxxxx",145,"",0,"",0
RING
+CLIP: "xxxxxxxxxxxx",145,"",0,"",0
You have to do it only once. When ever the modem receives a call (Incoming call) you get
RING
+CLIP: "xxxxxxxxxxxx",145,"",0,"",0
You can read this using microcontroller.
Read post#8 and #9 here.
https://www.8051projects.net/t59453/embedded-gsm-development/i-want-few-at-commands-sim300-gsm.htm