Get a free DID at IPKall here:
http://www.ipkall.com/Set SIP URI to : AshishF @ xx.xx.xx.xx:5061
xx.xx.xx.xx is the WAN IP address of your cable modem.
OR
Set SIP URI to : AshishF @ name.dyndns.com:5061
If you are setup on Dyndns.
SP2 of your OBi100 must be SIP and enabled.
Call your IPKall number and your OBi100 will ring.
I tested this on my OBi110.
Edit:
Forgot something.
5061 must be port forwarded to the OBi100 in you router.