Updates to software are usually done for the better, but sometimes it results in changes that are less than ideal. This is what seems to be the case right now for T-Mobile iPhone users who are discovering that after the update to iOS 11.3, their phones are starting to prioritize cellular calls over WiFi calling.
Users are reporting that after iOS 11.3 and the Carrier Bundle 32.0, their iPhones won’t use WiFi calling unless their cellular signal drops to a very low point, which is around one bar. Prior to this, cellular signal did not have to get so low in order to switch to WiFi calling. Whether or not this is a big deal depends on how much you rely on the feature.
For those that do, then this could present itself as a problem, although TmoNews recommends users putting their phone into Airplane mode, enabling WiFi and then make a call using WiFi if they absolutely have to. As to whether or not this “problem” will get fixed is unclear. So far T-Mobile has indicated to TmoNews that this could be an iOS problem, but Apple has yet to clarify the issue and how they might fix it in a later update.