I use hosted VoIP and have a mixed system due to offices that are dealers or franchises... Through careful programming and forwards, the systems function as one without coughs or problems. It's a mix of Verizon, Jive, Vonage, SuddenLink, Consolidated, and Windstream. The backbone is Jive and most of the data connections is Verizon, Consolidated, Armstrong, and AT&T. It works well and no one would ever suspect it was a collage. Jive works well for a remote office, just point it at the inbound trunks and point your current system at the jive, and set up false extensions. No one would know the difference... It just works flawlessly.
VPN's, Sonic Walls, and Edgemark routers all make trouble in your VoIP networks and should be avoided. Carry your crucifix to wart off the evil.
90% of most companies including Gov agencies use VPN, I've use Sonicwall from a Colo to 21 offices flawlessly with Avaya.
Edgemark does compression, VoIP is Compressed thus your problem BTDT.
I do VPN with Sonicwalls with Digium Phones, Users internet Wan port, Phone on Lan Port and it's also Wireless