Device: Xperia 10 II
Country: BG
Provider: A1
Date tested: 2025-11-02
Registered ✓
Voice in ✓
Voice out ✓
SMS in ✓
SMS out ✓
To achieve it, I enabled 4G calling (beta) in the phone settings, then did the following (while ssh’d into the phone from my laptop)
devel-su
getprop persist.vendor.somc.cust.modem0 # prints `S8827.1`
cd /vendor/oem/modem-config/
mkdir S8827.1
cd S8827.1
echo -n "mcfg_sw/generic/common/default/vl/global/mcfg_sw.mbn" > modem.conf
… and rebooted the phone. Voila! 