There's so much stuff going on with the network when you make a cellphone call... DTX (discontinuous transmission - signal is reduced to zero when the caller is silent), AMR (adaptive multi-rate codecs), Dynamic Half-Rate Allocation (speech frequency range is cut in half to potentially double the number of calls a base station can handle)... on top of that there's cell load sharing (the call is moved to another cell), frequency hopping (the call is moved between frequencies), intra cell handover (the call is moved to another channel within a subcell)... what you perceive as one continuous phone call could can be passed around hundreds of times between frequencies, cells, subcells and channels and processed with so many different codecs, no wonder it makes weird noises.
So maybe you've just been lucky with the network today. The only thing Apple could do about it is add a local, software-based noise gate to suppress all the clicks, pops and squeaks.