Imagine a basic cell phone which operates in only 2G network and able to provide voice call & SMS service. Does this cell phone transmit any data to BTS in idle time (that is when the cell phone is not busy with voice or SMS service)?
More details on the problem:
I have built my own BTS (OpenBTS) by using Ettus USRP B200 (at 900MHZ Band). I have two GSM Quadband 2G compatible cell phones, those I am using to send SMS and Calling purpose.
By using another USRP B200 board and with the help of GNURadio, I am capable of detecting the continuous downlink frequency (for my case that is 945MHz). So using the GSM ARFCN frequency calculation, the uplink frequency should lie in between 900MHz-903MHz. Within this band, the uplink frequency is easily detectable while the cell phone is active (i.e., while sending an SMS or making a call).
But, whenever the phone is idle, I am not able to detect any signal within this band (900MHz-903MHz). What should be the possible explanation of this phenomenon?