It’s definitely a major problem of the iPhone13ProMax, NOT the carriers. If you don’t live near an antenna but travel a lot like me, and bring like me an iPhone8Plus and an iPhone13 and use regularly the phones (don’t look only at the signal bars) you can experience very clearly that you have much many dropped calls and blind spots with the iPhone13 than with the iPhone8. I guess it is a problem of the internal antennas, not the software, probably due to the addition of many 5G frequencies: as you may know, it’s very difficult to design an antenna that works on many frequencies, and for sure it’s not optimized for any frequency.
The only solution in future phones would be to increase even more the number of internal antennas, each optimized for a small range of frequencies.
So even switching off the 5g in the phone wouldn’t help, you only solve the operators’ problems of the calls handover from 5g cells to 4g cells (calls dropped while moving quickly, due to slow handover process in the operator network)