Hello @vane171,
I never tried that one given its age. There are other free solutions just like it, and more modern like PortSIP UC, MizuDroid, Linphone.
It is not the good thread, but if you have registration problems, maybe you can try SSL (secure) connections. This reduces a lot of potential issues. Maybe the following thread can help:
As for paying for a softphone, I know. I was there too. I do not like to pay for such things. But what changed my mind is (1) the one-time payment, and (2) all other solutions I tried were either not reliable or draining the battery. Since there were quite good reviews for Acrobits/Groundwire, I tried it.
Now I would never go back, and if I had known it before, I might have bought it years ago. I now find myself using my android phone instead of my home phones. I paid a little bit more for Groundwire to have the easy transfer option. (And if you activate the family group of Google Play, up to 5 members can use the application for free too. )
Kind regards.