I’d Tunnel you’re traffic between your endpoint routers using GRE or GRE and IPSEC if you want security also. GRE is VERY to configure on Cisco routers and it’s an open standard so other router vendors should work too. Using access-lists to pick out your h323 traffic you want to encapsulate (i.e. VoIP traffic between two endpoints) in a GRE Tunnel that’s unencapsulated on the other side. No ISP I know of would block a GRE or IPSEC tunnel for fear their blocking “legit” data traffic.
If they are, simple, change ISPs.