Signal is working on a standalone version for Signal Desktop that does not require a mobile phone

I don’t have much to add here, though as far as I know, partially from reading the topic thread I’ll link below (which I suggest you give a look) there are VoIP options for use with Signal that work outside of the USA.

In my case, yes I have retained control of the VoIP number which is tied to a Signal account.

Reference:

• VoIP service recommendations