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This is regarding unlocked phones sold from stores. A major carrier in my country depoloyed a VoLTE network and it worked even on custom ROMs with very minor problems. Now 2 other carriers are following suit but they had to work with phone manufacturers to release an OTA update so VoLTE would be supported by the phones on their network too. Isn't VoLTE a standardized protocol? Why do phones need updates for another VoLTE carrier even though the phone supports VoLTE in another carrier. Isn't it just a matter of changing APNs?

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VOLTE is a standard. And like all standards the devil lies in details of implementation. Carriers may choose different implementation for various reasons

Section 2.2.1 of PDF document here offers reasons as to why implementation differs and hence updates are needed

....VoLTE interoperability issues might be related to EPS bearer set up, proprietary simplification of SIP call flows, operator specific IP headers, different security configurations IPsec, AKA etc.

You have detailed technical reasons explained subsequently in the document

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