I’m with H20 myself (they are an MVNO of AT&T). AT&T are switching off their 3G network in Feb 2022 which will directly affect anyone on a non-VoLTE phone after that point.
It’s not accurate that it’s an FCC mandate but it is accurate that the 3G network that H20 uses will shut down next Feb. My SO is still using a Nexus 5X and has had the SMS message you’ve mentioned. H20 do call it an FCC mandate but that’s not accurate - clicking through the H20 link from the text provides this link to the FCC regarding the issue - note that nowhere in that FCC page does it mention anything about it having mandated carriers to switch off 3g.
The point is that with AT&T deciding to make the switch, H20 have no choice but to do the same as they’re simply reselling service using the AT&T network.
As I mentioned on this thread I don’t have any problem with this not being implemented as it affects a market that Jolla doesn’t serve - legally speaking you can’t buy OS licences from the US and the Sony phones they work on aren’t sold in the US either - the LTE bands they support are very different to the LTE bands AT&T use. From that post, I mentioned that my XA2 supports LTE bands 1-5,7-8,20,38. The main bands AT&T has widespread use of are 12&17, so adding VoLTE support is pointless when my phone can’t connect via the bands it supports as they aren’t the main ones used by AT&T.
Right now the only option seems to be to switch to T-Mobile (or an MVNO that uses T-Mobile) and hope they don’t annonunce a shut-down their in-house 2g&3g network (they’re shutting down the Sprint 2g/3g network next spring) but with AT&T switching in Feb & Verizon in December 2022 I’d assum it’s only a matter of time.
The TLDR version of this post - Sailfish OS is unsupported in the US and even if they add VoLTE support, some, if not all, of the Sony phones supported on Sailfish X were never sold in the US either so don’t have the required LTE bands to work anyway, so adding VoLTE support may well be pointless.
Guess we have to hope that someone makes a phone that Jolla can support that supports the full global range of LTE bands that anyone may need.