Juniper Junos SIP – SIP ALG on Juniper SRX100H2 Configuration Guide

As @Ricky Beam indicated, you should have no issues other than delay with fully-functional, SIP-aware NAT devices. This is known as ALG (Application Layer Gateway) on some lower-end network devices and SIP Fixup or SIP Inspection on different Cisco firewall platforms depending on software version.

Generally, the recommendation on not using ALG comes from poor implementation of either ALG itself or the client not putting its address into the SIP packets for it to be found and updated with the NAT'd address. On higher-end network devices, this generally works though you may run into a bug as I did in a recent question with an older Cisco PIX version.

Any type of ALG/SIP Inspection requires symetrically routing especially if implemented in a stateful firewall so the NAT'd addresses can get reversed on their way back to the client sitting inside the NAT device.

Seems that the trick is like that (credit to Alex over at serverfault.com):

First go in your configuration terminal and create the class-map;

asa(config)# class-map SIP_5080
asa(config-cmap)# match port tcp eq 5080
asa(config-cmap)# exit

Then create a policy-map;

asa(config)# policy-map SIP_Policy
asa(config-pmap)# class SIP_5080
asa(config-pmap-c)# inspect sip
asa(config-pmap-c)# exit

Finally, assign the policy-map to an interface; asa(config)# service-policy SIP_Policy interface [name of your interface]