Jeff Liebermann writes:

I've been thinking of building (and selling) such a device. It can be
done if:
1. You have an RF direction finder.
2. You know the sub-band where to expect the vendors transmissions.
3. You know the SID (system ID) of the vendor.
4. You have a map or database of the vendors service areas.

You could also add some LTE/UMTS module, some of these can do a network
scan with an AT-command and give you the cell id, technology, channel
number etc. of all 'visible' base station.

Apparently even some USB dongles can do this, so you could connect some
cheap SDR+modem+GPS to an RPi and do your magic.

There are also cell tower location databases like Opencellid.

--
mikko