Usually you will want to do one or the other of those, not both. Sip proxy transparent is where I would start, without the port-forward.
First make sure your firewall is configured correctly for NAT, you can surf the web from the inside zone, etc. Then enable ip sip, ip sip proxy, ip sip proxy transparent.
Don't forget to put "media-gateway ip primary" in the interface configuration both inside and outside.
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