Settings ->Network & internet -> SIMs -> Tello -> Turn off VoLTE
Even though tello states in it's documentation that VoLTE needs to be turned on, I've found that having it on causes calls to drop immediately, but data and SMS still work. (Haven't found a fix for getting MMS to work consistently yet.)
I've played around with pretty much all of the settings there and it's the only one that seems to have any effect.
(Pixel 8a, Android 16, April 5th Security Update, 2026)