Detect cell phone sensor

Hi there,
I am wondering if there is any sensor or way how to detect cell phone.
For my project want to have different behaviour if the cell phone / smart phone is near by device or not. But have no idea how to detect it.

Thanks for ideas

Closest available Blynk method is GPS trigger

http://docs.blynk.cc/#widgets-sensors-gps-trigger

Otherwise you will have to Google around for BLE beacon or something like that.