VOIP, PPTP VPN Server, TrixBox, Freeswitch PABX

Hi there i m looking for person who has good understand of pptp vpn , Voip, routing calls, astrisks , Trixbox work description i have pptp vpn server on 2008 R2 with public IPs. and on client side i m using xp and gnugk as a gatekeeper to route calls on voip gateway from other lan card using private ip problem is i m getting oneway voice from some vpn clients and some are fine and even some works fine some time and other time same oneway voice issue please suggest me better option and quick as well and one more thing when i use vpn account which i took from reliablehosting it works on every client no oneway voice issue working 100%