Having similar issues with my iPhone 14 Pro Max on T-Mobile.
For example, today, I left my house and drove towards the highway (about 3 miles away). Directly outside my house, I normally get 1-2 bars of LTE. Normally, when I get about halfway to the highway, my 1-2 bar LTE signal turns to 1-2 bar 5Guc, and by the time I reach the highway it's a full 4 bars of 5Guc because there's a tower right next to the highway entrance.
However, my experience today was that my phone showed SOS directly outside my house. When I reached the halfway point (where my phone normally hops from LTE to 5Guc), my phone instead showed 2 bars of EDGE service. By the time I reached the highway, it was full 4 bars of EDGE. I pulled over, toggled airplane mode, and the iPhone seemed to snap out of it and showed full bars of 5Guc with 800Mbps download speeds. So, clearly not a tower issue.
I haven't fully documented past issues like this, but it's this sort of thing I keep running into. Off the top of my head, another example, last week I was staying at an AirBnB and noticed that when I first got there, I had full bars of 5Guc. I remember being happy about that. About an hour later, my phone stopped being able to load anything and I noticed I was only on 2 bars of LTE. Toggled airplane mode, and the full/working 5Guc signal came back and the issue did not repeat.
Frustrating to have to keep an eye on my phone when not on Wi-Fi to make sure it's still got a working signal, or else I could miss something potentially important, as I notice SMS/calling even has issues during these failures.
EDIT: I should note, last week my iPhone was on iOS 16.1 and today it is on 16.2 (installed the beta to see if a modem or carrier version bump would help with regard to these issues). So, the issue seems to persist regardless of version, for me.