Voice over the internet VPN cannot
produce good quality because you can and only have controll over QOS in your existing device
interface and after the packets leave you router is would not be given any preference or higher priority over the internet.
They would inturn face delays, jitter etc which willd efinately result in bad quality voice.
VPN span various hops via the internet to the remote device, hence QOS can only be guranteed in environment where you have control.
The isdn link might be a better solution for both voice and data. This would be a direct dial-up 128KB bandwidht to the headquaters
and hence you have control of both router at the HQ and telecomter site.
Hence the right QOS an be applied and voice quality will be greately improved.