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I think you are doing the bandaid the way I would. You didn't say what the are used for, only vital. If 911, may not be the best approach. Fax over it may not work also, don't know the trunking at the site they hang off of.
If you can call it from an extension, it should work. Have you tried using a know working DID temporarily as a test? How are the DID's handled? Do they match or are they altered via speed call table or other service?
Are you saying that you changed 1430 to point to this ne sip device and it works? If so, maybe 2656 isn't setup by the carrier correctly.
How is the DID routed? Is that DID [2656] definitely part of a block set up on the PRI? Sounds to me like teh DID is not routed on the Mitel.