I am sure many of us have heard mvno like cricket, metro, and visible. There are couple exceptions, but in most cases the customer that is postpaid will have data will that actually works when the towers are congested.

“This is a complex topic that pops up a lot so I thought that I would organize all of the available info in one place. One of the key differentiating factors between postpaid, prepaid, and MVNO services is data prioritization. Basically carriers manage the congestion on their networks by assigning a different QCI class to different types of traffic. For our purposes, we will only be looking at QCIs 6, 7, 8, and 9 but there are higher priorities that exist for things like phone calls that will be universal across all of a carrier's plans.”

https://old.reddit.com/r/NoContract/comments/oaophe/data_prioritization_policies_of_the_carriers_and/