As others have said, carriers do definitely "de-prioritize" traffic for some users. In general, this isn't about cross-carrier traffic (ie, AT&T v's T-Mobile), but as a way of differentiation between the carriers different plans, plus those from "MVNOs" (Mobile Network Virtual Operators - basically all of the carriers except T-Mobile, AT&T and Verizon who are in turn buying service from one or more of those carriers).

Exactly how they split things varies between carriers. however you can see a great summary of the current situation at https://www.reddit.com/r/NoContract/..._carriers_and/

However there's an important distinction between 'data' and 'voice' that's relevant to the OPs question. All of this de-prioritization only applies to "data", and it does not apply to voice calls. Voice calls get assigned to a different level of service that is higher than data, and will almost always result in the same quality regardless of whether you're using the underlying provider or an MVNO, and regardless of how your data traffic is being prioritized. (Technically in 4G/5G everything is data, but the data stream that is used for voice calls is what gets the different QCI level). So the answer to the question about "phone call quality" is that no, that will not vary based on the users identity, but data transfer speed/latency very certainly can!

As far as I know there are still some pockets where T-Mobile will roam to AT&T. Plus you've got areas like Alaska where both will roam to another local carrier. They also sometimes enable such roaming during/after disasters when networks are impacted. But yes, it's certainly not a normal situation.