It's always possible that they're blocking the ports you're trying to use for VoIP. A VPN would get you around a blocked port with the penalty of increased bandwidth and processing overhead - in other words, it might work but it's not a scalable solution.

"There is one thing and only one thing in which it is granted to you to be free in life, all else being beyond your power: that is to recognize and profess the truth." - Leo Tolstoy