Here is a Cisco doc/link:
http://www.cisco.com/en/US/partner/docs/ios/voice/cube/configuration/guide/vb-gw-sipsip_ps10591_TSD_Products_Configuration_Guide_Chapter.html#wp1345256
It is for version 15.0 but applies to 12.4 and 12.4T as well.
We have used this with our local carrier. Just make sure your carrier supports the Options Ping Out of Dialog.
HTH.
Regards,
Bill
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