Best technology encrypted communication over the internet?

We are talking about implemetning a system for multi site communication over the internet. Basically the long story short is that we have clients with restrictive outbound communication, but we need some way for our applicance to communicate robustly to a central system. By restrictive outbound communication, I mean that these sites have firewalls, proxys, etc that may allow only one port out. Typically we mandate that 443 remains open, but that cannot be guaranteed either. Most of the clients are hospitals so encryption of the packets is important as well. That said we may be able to piggy back on an existing VPN connection in most cases, but whatever route I do needs to be flexible enough to allow alternative connections. Typically I have used WCF service calls for things like this in the past, but Ive never used WCF for communication outside of a LAN. My thought was that a simple WCF service would work, with the packets encapsulated in a SOAP or JSON object. But I really have no idea what the ideal solution would be in this case and Im just curious what you all might do?