Weird issue with remote users

Hi all,

We have an Asterisk-based system for our company PBX. We have no issues with quality internal (SIP-to-SIP), or external (ISDN PRI-to-SIP). We use Digium Wildcards and pretty heavy duty hardware.

We also have several users in the field who use a regular SIP deskphone, connected to our network via a VPN appliance (custom built). If they call us (or we call them) there is no problem whatsoever. However, if they call an external landline phone, then the call quality is extremely poor.

To illustrate, this is the problematic path:

SIP Phone <-> VPN Device <-Internet-> VPN Device <-> PBX <-> ISDN PRI <–PSTN–> Landline

However, these two paths have NO problems whatsoever:

SIP Phone <-> VPN Device <-Internet-> VPN Device <-> PBX <-> SIP Phone

SIP Phone <-> PBX <-> ISDN PRI <–PSTN–> Landline

This is quite puzzling since we know the WAN connection is fine (since internal calls are fine), and we know the PSTN calls are fine (because internal users call call out fine) - it’s just when done together…

Anyone have any ideas?

Thanks in advance for your help!