No Internet / mobile data while phoning

[peterleinchen]

Not fully true or always the case (which I needed to learn as well.
But in general I’d say this is closest to true.

See here

VoLTE support in SailfishOS?

Also false, but data over 2G is already not very much use. Combining CS and PS puts even further limitations of the rates you can achieve (and isn’t always supported) but technically, it works.

and following posts for details.