Just posting this idea for anyone who may be interested, as I just did this myself.
I've had a number with VOIP.MS for about a year - it is incredibly cheap & I had it mapped to my SIP phone.
Upon realizing that it had been using .85/month when the initial deposit ran low, I added a bit more, then chose to add it as a 3rd line to my Obi box - which SHOULD have been trivial to accomplish.
But it wasn't, and after poking at that for days I gave up trying.
As it happens I've got a superb, non-activated Kyocera Hydro that was my Twigby phone with Android 4.4.2 just hanging around that gets a little use as a camera - and of course it is usable via wifi only and has the native SIP client.
Setting up SIP on the phone was trivial - it registered right away & tested fine for outgoing.
Incoming was a bit more effort, but once I got the info only took a minute to fix, then tested & setup VM which comes with email notification at no extra charge.
The sound quality is good IMO and it works just as a phone should.
Now I've got that number working on a totally portable phone which may end up costing ~$20/year with my low usage & can be set up to grab calls from my extra GV number that has been in limbo for quite some time since the Simonics gateway for GV went away.
If you have the desire or need for some added & very cheap services, I can recommend VOIP.MS without hesitation, and they even have a referral program - though I haven't looked at its details.