skyplonk: I have mentioned on a couple of other threads here that we had this exact issue with Spark and S21FE (and a couple of other phones) for a year. Happy spark customers for many years. All was good until late 2021 for us.
It was looked at by a range of spark staff including members who also are active on Geekzone. But ultimately was never resolved and after nearly a year ported to Vodafone (it was affecting the ability to do my job), without anymore issues.
I had constant dropped calls and/or calls with broken audio or calls that would just fail to setup or incoming calls that would go directly to voicemail.
It was not everywhere which was odd. Spark took call traces and could see packet loss on the uplink and an issue with a tower(s?) In our area.
I had to disable VoLTE but that option was taken away with a software update. I found using the app "Samsung band selection" from the app store I could turn it off via a hidden menu. That was all well and good until I was in a RCG/LTE only area.
The same phone on vodafone was ok and all you did was change sim?