Wade:jonathan18: Thanks for all the helpful posts. I've been off experimenting with a couple of the apps, and they were totally intuitive to set up. It looks like Lollipop has no built-in SIP phone client, so a third party app is needed on my phone. I've Tried the built in one on my wife's phone running 4.3 but it doesn't register with the settings I've tried; not nearly as straightforward as the other apps.
TBH, I find the design aesthetic of the 2talk app really tacky! It also doesn't seem to have any easy ability to close the app (not that I've found) but Adore, while a nicer app to use, isn't registering properly upon rebooting the phone. Will give the other app mentioned a go as well to see if that works better for me.
Thanks again.
You might be right on the Fritz!App being WiFi only, I reread a thread i started in the 2D forum on Home Phone Plus and this was mentioned, however another person had managed to set up via SIP to work over mobile networks. I'm booked to switch providers on the 17th so will report back once I've (hopefully) made it all happen!
In the Fon app, goto, Settings, Advanced, 'In all networks' and check the box (off by default). I believe you will need to enable external access on the Fritzbox as well. Full disclosure, I have not tested this (yet).