Right now I still have a landline, but I’m considering switching to VoIP.
I found a cheap VoIP provider (less than €10/year for keeping my landline number), but they don’t offer software. They recommend Bria Mobile, but this App seems to be subscription based (for just the App, no VoIP service) and with €0,99/month the App is more expensive than the service itself…
you can install it on your iPad as well. Works as normal, but not full screen.
If you look in the AppStore there are many iPad VoIP clients. Be careful as most tie to a specific service. I use Acrobits as it allows me multiple accounts