I was going to post something similar.

T-Mobile will require VoLTE for all phones starting January 2021, AT&T to follow suit in February 2022

In the US, T-Mobile and AT&T are the only national GSM carrier choices (and in the end, US GSM MVNOs generally use T-Mobile or AT&T's network). Unless Sailfish adds VoLTE support (which may not even be enough if this requires carrier cooperation), this would force me to switch to another OS.

Yes, I know, Jolla doesn't support the US market so I should not expect anything. I understand that, but it will still be a little sad for me if/when the day comes where I won't be able to use Sailfish anymore, which has served my needs/interests quite well these past few years.

Edit: Didn't see that this was posted to the N900 forum, but the concern is relevant for multiple devices and OS's running on T-Mo US or AT&T