I just check out Mediaring Mobile VoIP website and to the contrary, Nokia N95 is supported and so is the newer Nokia E71, which based on the various reviews, will be launched early next month. The E71 looks sleek .. hmm .. maybe I should get one when it's out :-D
I've tried the service out yesterday with another friend using N95 (I'm using E51) and we are able to make free calls to one another. So I believe it's not true that you can make free calls to one another "if you are using the same client" as what you mentioned.
Whether you are using Truphone or other sevices, they will also advise the user to check the phone's firmware before signing up. That said, the perogative is of course on the user and most users will conveniently skip that. I for one, did not check but my E51 works fine with the factory installed firmware out of the box .. Well, again, being my lazy self :-P
Ok, gonna give this service a spin and see how it goes.