JakeD presented the following explanation :
> On Sun, 23 Mar 2008 10:02:32 +0000 (UTC), Gordon Henderson
> <gordon+usenet@drogon.net> wrote:
>
>>> Is there a VOIP phone service that can be used via a mobile phone?
>>
>> I suspect you really want to ask the question: Is there a mobile phone
>> that can be used via a VoIP service ...
>
> I can't see any error in my wording of my question. I like the VOIP
> service I use (at home), because, unlike Skype, (as I understand it),
> I can make calls to any landline or mobile, and any landline or mobile
> can call me. The VOIP service is much cheaper for making calls abroad
> than is the mobile phone service. So what I'd like to achive is to be
> able to make calls via my VOIP provider, using my mobile phone,
> without getting billed for a call by the mobile phone company. This
> may be impossible; I don't know.

It's not impossible.

I've used fring on my e61i to connect to voipcheap/sipgate etc. Whilst
the Vodafone 3g connection was useable, the latency was higher than
when using wifi.

>
> I'm not very familiar with wifi internet access, at wireless hotspots,
> but I was hoping that there was a way to make the call via the VOIP
> service, once one is connected to the net (free of charge) at a wifi
> hotspot. Perhaps not.

You may find that you can ring out OK, but inbound may fail.

Or, you may find that you suffer from one-way audio issues.