@Josele13 In response to your question about the dual sim adapter: it will definitely work, but I don't think it does what you expect. Dual sim adapters like this allow you to connect two SIM cards to your card slot. The phone doesn't actually work with both cards simultaneously. Instead one SIM is operating at a given time while the other is unusable. You can alternate the two by turning the phone off and then back on. Not a great improvement compared to carrying the second SIM in your wallet and changing it when necessary. It's basically only a convenient storage place for the second SIM. Not to mention the adapter is pretty big and I doubt it would fit in most phones.
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RE: VoLTE and 3G shutdowns
Is VoLTE support planned for UT anytime soon?
It was discussed on a Q&A a few weeks ago, and also on the Telegram Supergroup. We also have a thread about it on the forum here. The core developers are aware of the need, but support will probably not come soon.
Dobey mentions in the forum thread that getting to VoLTE will take significant work. Without meaning to put words in the mouth of the core development team, the chief priority of current development would seem to be getting to the Unity 8 cutover. After that is complete, many other things can be worked on, but there's no telling when VoLTE will get to the forefront of the priorities.
Fortunately, 3G retirements are not yet widespread. In the US, Verizon will be VoLTE only by the end of 2019, but other US carriers have longer timelines (2022), and while I've read of various Asian carriers wishing also to retire 3G, I don't believe that will be happening right away either.
What sort of things need to be done to get VoLTE support? How would I get started contributing to it?