If you will connect the VPX IPPBX with VG3XE-FXO gateway [...]
VG5X VoIP Gateway is one product of VoIP VG series by VOPTECH.Carriers and enterprises can use it to provide regular phone, fax, and IP Centrex services through broadband networks.
The VG4X high-density FXS/O gateway series enables businesses of all sizes to create an easy-to-deploy VoIP solution.
VG3XE features the ability to connect standard telephones and fax machines to IP-based data networks with the additional benefit of an integrated connection for legacy telephone network .
VG1X 4-8 Ports Gateway powerful hardware platform provides sufficient power for IP packetization, voice compression and echo cancellation even when the voice traffic is at the peak.