Satellite detector and a mobile phone
The satellite detector is looking for IF frequencies from the LNB, which are typically between 950 - 2150 MHz, depending on how you are powering the LNB (13v or 18v, with or without 22kHz tone). Many mobile bands are also in this spectrum, except the super-high LTE bands e.g. 7, 30, 38, 40, 41.

Without any attachment to the LNB the unterminated F connector on the satellite finder (or if you have the coax attached, the unterminated end of that) acts as a poor antenna, but effective enough to pick up the strong signal from your mobile device.

When the LNB is attached, the input port to the satellite finder is now correctly terminated with 50ohms and will not act as an antenna. As alex.forencich correctly said, the LNB is filtering only for Ku-band signals from the sky, and the IF frequencies will be filtered out in its RF front-end.

You can use a bias tee (you will also need a capacitor to stop the 18v DC from going into whatever ADC/decoder you plan to use), but the scenario does not seem to make sense - the LNB is not useful for mobile signals, the satellite finder itself is not providing you with any gain, and so the signal you will retrieve from the satellite finder output will still be a low-level mobile signal requiring down-conversion and decoding (and likely amplification and filtering). Do not underestimate the complexity of decoding a modern mobile signal... 2G/GSM signals might provide the most interesting starting point due to their time-division nature, you might be able to see it on an oscilloscope once down-converted to baseband.