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 producesPDN_IPV6_CALL_DISALLOWED(0x7f3)from the Sunrise network on every IMS PDN attempt. -
Type: change
ims→ims,cbs(or any combination includingimsplus another type that doesn’t conflict with another APN — i.e. anything exceptdefault,supl,mms, which are already claimed by the Sunrise Internet and MMS APNs). Withtype=imsonly, iodéOS 7.5 doesn’t actually request the IMS PDN.
Steps: Settings → SIMs → APNs → IMS → set protocol to IPv4, set type to e.g. ims,cbs → save → reboot.
After this, IMS registers and calls should work again.
Part 2 (optional): Yallo side (the carrier-fixable part)
After the APN fix, my outgoing calls were still blocked at the network with a “Access to the number you dialed is barred” voice announcement, SMS sending didn’t work, and supplementary-services MMI codes returned generic errors. After contacting Yallo customer service a couple of times, they did another reset/re-provisioning on their backend, which restored outgoing calls, SMS, and supplementary services.
Thanks to everyone in this thread for the cross-device test data and @rik for engaging with the diagnostics. Hope the APN workaround above gets you up and running quickly too.