Mobile Phone Detection

Hi all,
I would like to start a new project in which i need to detect the presence of a mobile phone in a area. I have been investigating the resources available in the SDR community and i see two vias: detect gsm/lte packets or detect power sent by the mobile phone.
Packet detection:
I find gr-gsm and openlte: project that can detect mobile cells but i don´t know if they can detect broadcast packets from the mobile phone.
Power detection:
I also consider the possibility of detecting the power of the mobile phone using a power scanner but i see that the bandwidth is limited and as the gsm/lte signals are burst, i see difficult to detect them with the scan times that for example i see in the RTLSDR Scanner application.
I am a little bit lost: ¿can someone advice me or give me any idea to detect a mobile phone within an area?
Thanks,
Julen

When a mobile phone is connected to a Base Station/Cell/NodeB and is not in voice or data call, it gets/send some information periodically. This period can vary from technology to technology. You can utilize this property to identify a mobile phone.
While connected to voice or data, power will be high but bursty in nature (as you mentioned) in case of data call and power level will also depend on BW. Again detecting UMTS signal will be more difficult as they are noise like over a large BW.