Hi Pretended6910 and welcome to the community forum.
Can we clarify the problem(s) please?
Are you having difficulty hearing your caller?
Does your caller have difficulty hearing you?
Do the problems occur when using the earpiece (default mode when holding the phone to your ear) or when using the speaker mode?
Please make sure you are not covering a microphone accidentally by the way you’re holding the handset. There have been problems in particular with the mic 3 which is halfway up the left side of the phone, which may get covered by the base of your thumb if you hold the phone in your left hand.
The FP4 should function correctly with all carriers in Europe. iD is an MVNO which means they commercialise services but actually use the technical network of another company (Three in this case). While they publish a list of “approved” devices, that will just be the models they’ve tested.
I know of FP owners who use Three with no problems.