Most Reliable BB for International Roaming?

[31G]

The owner of my business uses a Bold via AT&T Wireless configured for BIS.

He is based in the U.S. but leaves several times a month and travels all over the world--St. Barthes, Hong Kong, Moscow, France-- and complains frequently about "call failed" errors, in spite of full EDGE or 3G signals, not receiving emails, "attachment unavailable" errors...

He'll have problems even as his assistant, who uses an 8320 via AT&T Wireless configured for BIS, is running smoothly standing in the same room. He's even been through more than one Bold and they've each had the same issues.

Usual solutions like reregistering on the network, pulling the battery, aren't helpful. In any case, he's the owner and it's imperative that voice and email downtime be kept to an absolute minimum.

Obviously all devices require troubleshooting, but the thrust of this inquiry is: does anyone have a recommendation for a Blackberry model or mobile service provider (or combination thereof) that works the MOST RELIABLY while roaming internationally, voice and data? I'm not talking about strong service coverage, just proper functioning provided sufficient coverage! Is there anything I can try preemptively to prepare a BB for international roaming? In people's experience, do emails come through more reliably while roaming internationally if configured for BES?

Any thoughts or suggestions would be appreciated.

Thanks!