This is fairly easy. You just NAT your SIP traffic to from your ASA to the IP address of your WAN port of your UC540. Your SIP provider may need turn on far end NAT support for this configuration because you may get some 1 way audio if not.
I've also seen partners put the UC540 and the ASA side by side. They would route the data out one of the LAN ports connected to the ASA and would put a public IP address on the WAN and route the voice out of that port.
Side by Side ASA and UC560 is very easy to do. Simply plug your WAN port on your UC5xx to a port on the ASA and set the switchport to access mode on vlan2 (default wan vlan on asa). Give your UC560 a public IP address in your scope and presto you are off and running. No need to setup a NAT on the ASA as putting the switchport on the ASA in Access Mode on the Public VLAN will take care of this.
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