A quirk with SKT VoLTE on iPhone 6 Plus

For the past two weeks, I've been having a strange problem when attempting to place calls with my iPhone 6 Plus. Many of the phone numbers that I know for sure exist would be met with "the number you dialed is not in service". If I switch the "Enable LTE" option in Settings app to "Data Only" instead of "Voice & Data", the phone calls would go through just fine. So it had to be an issue with VoLTE.

I first contacted my carrier, SK Telecom, to see if this was a problem at its end. But nothing turned up and I was eventually advised to contact Apple instead. Sure enough, colleagues using iPhone 6 on the same carrier did not have the problem. Digging deeper, I discovered that the problem did not occur only if I had country code prefix (e.g. +82 for Korea) on the phone number. So while +82-10-xxxx-yyyy would work, 010-xxxx-yyyy wouldn't.

Seeing that it might be some sort of a weird software issue, I contacted Apple support about this. Unfortunately, no clear solution came up, either. As a last resort, I was told to do a complete factory reset and see if that helped. As the troubleshooting was reaching a dead-end, I got a carrier settings update as I opened up the "About" page in Settings app. It updated the carrier settings from SK Telecom 18.x to 19.1. I told the support that I'd see if this update did anything, and if not, I might try the factory reset.

I attempted several calls to various places with the VoLTE setting on, and I saw that all of the calls now made through regardless of the country code prefix. The update apparently fixed the problem. It was indeed a software problem, but it was somewhat caused and fixed by the carrier. I reported this success to Apple support.
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