Actually, your circuit has not received the weak incoming call to your cell phone. Your circuit has receiver the strong transmission from your cell phone when it transmits that it is turned on and is ready to receive a call.
I don't know if cell phone transmit and receive frequencies are in pairs, like channel 248 and channel 249. Then if your cell phone transmitted on channel 249 you would know that the incoming call was on channel 248.
If your cell phone uses digital Frequency Spread Spectrum technology for transmitting and receiving then it uses many frequencies during each call.