Yes that should work and it seems you have answered your question yourself.
To make it more sophisticated you could register the IPKall DID with e164.org and call her for free from almost everywhere using SIPBroker.com or from any ENUM-enabled ATA worldwide...
Vice versa: Register your US Phone number with e164.org, your mother will dial the number she is used to and the dial plan you created in her Voxalot account will perform the ENUM lookup... The side effect will be that you can be reached for free by many others as welll!
Got it?