Three also offered free access to Skype. It used (uses?) a Three-specific Skype client (available for many, but not all, models of phone) to do the presence and signalling over data, and the voice over the normal mobile voice channel:

https://support.skype.com/en/faq/fa10976/how-can-i-get-skype...

To protect revenue, SkypeIn was blocked, and SkypeOut was restricted to non-UK numbers.