Thats nost likely because you are running IOS 15.1 and above. There is a feature called Toll fraud prevention that will reject any call coming from a device that is not in the trust list of the gateway. When you configure a destination pattern with a session target ipv4, the ip address gets added automatically to the trust list of the gateway.. Refer to this doc for more details
https://supportforums.cisco.com/docs/DOC-12228
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