Is there any VoIP service that works over squid proxy? For example, Skype and Gtalk works over a squid proxy but there are many limitations to deploy in business environments. Looking for the options to link with Asterisk or it can just be a simple VoIP gateway over a squid proxy.

[VoIP Service Provider]--[Internet]--[ISP]--[ISP's Proxy]--[EndUser's PC]

[VoIP Service Provider]--[Internet]--[ISP]--[ISP's Proxy]--[EndUser's FXO ATA]--[Non-IP PBX]

[VoIP Service Provider]--[Internet]--[ISP]--[ISP's Proxy]--[EndUser's IP-PBX]

ISP's Proxy allow only TCP 80, TCP 443, TCP 21 for the outgoing packets.