There's an incoming dial-peer on the router which matches all incoming calls (unless a specific DID has been configured) and rejects the call. The dial-peer has "permission term" configured. This is part of the toll fraud / anti loop mechanism which configured the router to only accept incoming calls placed to numbers which are configured on the system.
Remembering that the Unity module is a separate SIP UA, calls from CUE destined to PSTN would fail because of this, so what they've done is prefix incoming calls from networks described by access-list 2 (i.e. the CUE module) with ABCD. We then have an incoming dial-peer which matches calls starting ABCD, this is removed and the call is sucsessful.
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