Mobile phones should be time multiplexed.
(https://en.wikipedia.org/wiki/Time-division_multiplexing has GSM mentioned)
(3G and 4G use more compliclated multiplexing, but i don’t think they transmit and receive at the same time either)
For truly simultaneous tx+rx you would probably need two antennas and two frequency bands.
Receiving on a transmitting antenna must surely be one of the more impossible tasks.
Edit: It seems i’m wrong and they do rx+tx at the same time on the same antenna.
RF circuitry is almost always shielded with cans. But cans are rather expensive in regards to manufacturing. Large product numbers of course make them a bit cheaper.
Could almost imagine you @abcd567 having electrical devices in the receiving plane of your antennas is a factor in needing the filters you need. (But probably it is just the cell tower on top of your building especially if you are rather high up in the building)
the tale of the usb hub
One USB 3.0 hub in the house was completely annihilating 2.4 GHz WiFi for me, that was hard to debug because it would only be relevant when there was data transfer over the hub, for example a USB 3.0 hard drive being accessed via the hub.
I’m not sure what that hub would do in regards to ADS-B, but my antennas always were at least 4 m higher than the hub and with a cantenna or spider, they really don’t receive pretty much anything from below them.
(Anyway i used some aluminium foil tape to coat the interior of the USB hub housing to quiet it down.)