This is a hack, but I think it should work: set your bandwidth on the EXT interface to your actual ISP bandwidth. Prioritize VPN traffic, and give it a guaranteed bandwidth of whatever you need. Set the bandwidth on the INT interface to the same as the ISP bandwidth. Setup QOS for VOIP on INT with guaranteed bandwidth less than or equal to what you set the VPN bandwidth to.
The problem with this is if you have more than 2 interfaces, it will probably slow down internal traffic between the different subnets/DMZs. A workaround for that might be to set the INT interface bandwidth to 100mbits or 1gbit (whichever is appropriate) and still setup a max for VOIP with bandwidth less than or equal to the VPN reserved bw.
This is a hack, but I think it should work: set your bandwidth on the EXT interface to your actual ISP bandwidth. Prioritize VPN traffic, and give it a guaranteed bandwidth of whatever you need. Set the bandwidth on the INT interface to the same as the ISP bandwidth. Setup QOS for VOIP on INT with guaranteed bandwidth less than or equal to what you set the VPN bandwidth to.
The problem with this is if you have more than 2 interfaces, it will probably slow down internal traffic between the different subnets/DMZs. A workaround for that might be to set the INT interface bandwidth to 100mbits or 1gbit (whichever is appropriate) and still setup a max for VOIP with bandwidth less than or equal to the VPN reserved bw.