VPN from softphoen to softphone

[dwhipple]

HI, I have a client interested in setting up softphone to softphone.. When I talked recently with PFsense I was told a USB modem may be required,

What is the best way to do this softphone to softphone over VPN.    What do you suggest for modem. They have over 300 clients and want to be able to this on all phone calls.