[andy]

DialNow uses ordinary landline access numbers. I didn't necessarily intend to suggest it for use within the UK, as you'd still have the cost of the access number at the SIM's standard tariff for it, which is more expensive than some of those int'l tariff options.

When I suggested DialNow, I'd understood that you might be roaming Europe with a UK SIM and maybe Vodafone Passport, O2 My Europe Extra or 3. When roaming they don't have cheap calls to Canada and USA, but calls to UK (Vodafone) or within Europe (O2, 3) are cheaper, hence the suggestion. If you're likely to decide to get a global roaming SIM instead, then I'd overlook DialNow (it might be useful with some single country SIMs though, those with cheap local calls rather than those designed as an mvno for int'l calls)

I don't know if you can add the T-mobile bundle or top-up while abroad; I don't recall anyone saying they managed it, but I'll wait to be corrected. I don't think the UK networks accept foreign credit cards themselves, though a shop here would

Any callback service, or indeed any other phone call, will use the tariff applicable to the destination number, not to where that phone happens to be at the time.