iodéOS 7.5 – VoLTE broken with Sunrise network (Switzerland) – proven carrier-specific, not device-specific

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For anyone else that gets here, a solution to the Sunrise has been written up here:

Calls not working on FP4 patch 7.5

Resolved: full fix description for future readers Calls are working again. The complete fix was two-part: an iodéOS-side APN tweak, plus a carrier-side re-provisioning by Yallo (in my case). Part 1 — iodéOS side (the easily fixable part) Default iodéOS 7.X ships the Sunrise/Yallo IMS APN with settings that don’t work on this network. Two changes are needed (one alone isn’t enough): Protocol: change IPv6 → IPv4. Default IPv6 produces PDN_IPV6_CALL_DISALLOWED(0x7f3) from the Sunrise network o…