From the EE blurb this appears to be different as it is an extension of the wifi calling and uses your existing EE mobile number. There is no additional cost to either the subscriber or the person phoning them, outbound calls coming out of your allowance or at your standard rate.
Apple has provided a similar hand-off between devices on the same wifi for a while. It annoyed the hell out of me when the iPad rang for phone calls so it got turned off sharpish.
(I have no connection to EE)