International roaming and VoIP

Does anyone know what proportion of carriers' international roaming calls is accounted for by long, outbound calls? It strikes me that it's probably quite high - business people phoning into conference calls, ringing back to head office or to clients - or even tourists phoning home.

I think this section is uniquely vulnerable to PC-based VoIP substitution, unlike inbound calls transferred to the user, quick outbound calls for voicemail and so on.

At a conference the other day, a substantial section of the audience (admittedly attending a Mobile VoIP event) had been using Skype or some other form of VoIP over in-room broadband or hotel WiFi whilst they had been there.

I did myself - I had to have a 30min+ conference call with someone in the US, while I was myself roaming in Belgium. Given a choice between my mobile, the hotel phone, or the (relatively painful) €20 WiFi charge + pennies on SkypeOut, it was no contest.

Add in the fact that the timing of the call is often known in advance, the user (especially a business traveller) will likely have a PC anyway, the desirability of having web access to Google or a client's website while on the phone, and the ergonomics of a headset, and it's a bit of a no-brainer.

What's not clear to me is just how much of a typical carrier's voice roaming falls into this category - but I suspect it's a pretty sizeable chunk.