What did I do to get free calling?
1. I bought myself an Analog Telephone Adapter (ATA). I choose the Linksys PAP2 with two phone connections.
2. I registered at http://www.sipdiscount.com and ordered EUR10 calling credits. This Voice over IP (VOIP) provider offers free calling to landlines in most Western European countries and the US plus some really good rates for other countries (I call Suriname for 18 Eurocents/minute). I also received a free phone number.
3. I configured the Linksys PAP2 with my account and servers at sipdiscount.com
4. I configured my ISA Server firewall to enable SIP from my home network (more about this later).
5. Then I connected a phone to the Linksys PAP and voila!
VOIP calling is no different from ordinary phone calls. The quality is the same. Only beware that fax through voip does not seem to work very well. In the Netherlands it is possible to move your phone number(s) to voip. This service is not offered by sipdiscount.com, but I found it available at www.budgetphone.nl.
Now free calling is working, I started my migration away from high phone bills to cheap voipcalling. I will soon drop my ISDN line and move all my phone numbers (not my fax number) to a voip account.
I will follow up this posting with some more information about how I managed to configure the ISA Server to enable SIP for VOIP calling.