The protocols VOIP use are often not firewall/NAT friendly, and I suspect most hotel Internet connections will run you through one of both of those.

It could even be a case that only one person can run VOIP, and then any further connections will fail.

I've had mixed success - it works in most hotels, but there's a fair number (including the Sheraton on the Park in Sydney as I discovered last week) where it doesn't.

If you have the option, try using a VPN to connect to your companies network and use VOIP from there - that might work if your company allows VOIP traffic, although as you'll be running via another site it may drop the quality.