KevinRoach:
It can send it once every 15 min or more.
No, you misunderstand. Send where? Your phone is a client and cannot be guaranteed to be in range and listening to receive the data at the time the data is being transmitted. For that, you need a server that is listening 100% of the time.
You need your phone to receive the data on demand which means that either: A. the sensor is itself a server, available 100% of the time, which has implications for it's batteries, or B. the sensor sends it's data to a separate server which is mains powered, such as a cloud server or a raspberry pi.
Is this making any sense yet?
Note: when I say "100% of the time", no server can ever truly be available 100% of the time. I just mean close enough to 100% to make you happy.