T-mobile does have a hard 4 hour single call duration limit
After doing numerous tests with my T-Mobile service since obtaining it back at the beginning of May 2015, and doing a little research, it seems T-Mobile's network as a whole does have a single call duration limit of 240 minutes or 4 hours. In other words, if you are on a call, no matter if it's T-Mobile to T-Mobile or not, the call will always drop around 240 minutes into it, and you or them will need to call back. If using wifi calling, the duration is 4 hours exactly, but I've found if using the cellular network, the maximum call duration is slightly less than 4 hours, usually between 3 hours 51 minutes and 3 hours 55 minutes. Most users I'm sure will never hit this limit on a single call, but for those times where you may want or need to have a long chat by phone, such as when I was trying to troubleshoot a friend's server issue a few night's ago, it's good to know what the limit for a single phone call on a provider is, since it's never documented. I've done tests on Verizon, AT&T, T-Mobile, and magicjack, and can report the following results. T-Mobile as stated above has a maximum single call duration of around 4 hours, MagicJack has a maximum call duration of 1.5 hours or 90 minutes, and both Verizon and AT&T are truly unlimited, as I've had phone calls up to 9 hours, 10 minutes on Verizon and over 7 hours on AT&T. I also have google voice, and so far, I've had a 3 hour 5 minute long telephone call via Google voice, but I need to do more tests to see what the limit of GV is, if any for a single call. I do wish providers if they choose to have a limit per call, would document this limit somewhere, rather than making users wonder why their calls drop after a given amount of time. After all, they do advertise unlimited voice minutes, so they should in my view state there are maximum per single call duration limits. One nice thing about the VOIP provider call Centric, is that you have a single call duration limit, but it can be configured on a per user basis. So mine I think is currently set for 1 hour, but I can adjust that whenever I please all the way up to a max of 8 hours. And, there are other providers, such as vitelity, that are unlimited. As I learn more provider call duration limits, I'll update everyone.