Pixel 6a cellular connectivity problems - calls to some phone models (older) won't go through

I just bought a new Google Pixel 6a (recently priced lower than pixel 5a) and after a few days of use I am starting to wonder if there is a fundamental system (either hardware or firmware) flaw with Pixel 6a: I am NOT able to connect to certain phones. The symptom is very similar to "poor network connectivity" but it's only on my Pixle 6a. When I try calling my room-mate who has a rather older Samsung model, majority of the time it goes to voicemail without connecting at all while he is in the house and is able to receive calls from all other phones (including my old Samsung s7). Same in either direction. Other phones we use (all Samsung) and my other room-mate's pixel 5a do not have this problem. We've carefully verified it, very clear and repeatable problem verified on two older separate older Samsung model phones. Both were mostly unreachable from my Pixel 6a going straight to voicemail or silence and call dropping all together. We've troubleshooted with T-mobile and tried switching the phone preference from 5G to LTE etc. on the phone preferences, no difference. I have been looking around but don't see anything specific on this. Anyone else have cellular wireless connectivity issues with Pixel 6a?