[Stu]

I don't see a conflict between callbacks and data for the reasons mentioned by DN. It should be noted that callbacks are not limited to prepaid. Several Hong Kong providers offer their subscribers better roaming rates when using their callbackservice.

From what I've read, voip using an efficient codec gives you roughly ten minutes of talk time per meg. The average data roaming rate that I've seen is ten to fifteen US dollars a meg. (Come back Mega No Limits!!!).

Assume the lower rate and no termination rates, you are still talking about US$1 a minute over VOIP.

VOIP over 3g is also real shaky. I've done it on one provider in the UK and to quote my British friends "it was simply brilliant." In other places, this isn't likely to be the case.

By the way, for those of you using SIP and Windows Mobile 5 or 6 (without the SIP module) take a look at Fring. It has great connectivity to both Skype and SIP. It also gives you Skype on Symbian.