Free calls to the US using Gmail is well known but I find it's a hassle to use a PC to make phone calls.
Having a SPA2102 that I can't use as I had planned I looked around for other uses. I then found this thread.
It took me about 5 minutes to setup and now I can make free calls to any US number from a standard handset connected to the SPA2102.
Details left to the reader but you do need
a US SIP provider (I use sipgate)
A US Google Voice number and account (can get that using sipgate first to get a US number and a VPN to get over GV's checking)
I am surprised how easy it was to setup and the high WAF! SHe wanted to call a friend in NY today to check she was not affected by the floods and making the call was so easy.