When you say you cannot receive calls does that mean one way audio or phones not ringing at all?
If they are not ringing it would be something stopping the traffic on the ASA. I would allow all traffic from IP addresses of your sip provider to the public IP (the NAT-ed) address on the ASA for the sake of this test. Then go onto the UC and do a debug (debug ccsip messages) and see if something comes up when you dial the SIP number.
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