VOIP over VPN is not a trouble. We do it every day to bypass ISP ‘s on countries where the normal VOIP ports are being blocked.
As how to setup that I.P. phone to that switch,the best advice will be take the phone to the same location where the Talkswitch is at and make it work there.
Once it works localy, put it trough the VPN. It should work the same .
Best regards.