Application Layer Gateway: Using an ALG involves building intelligence about specific protocols into a firewall so that it can make the necessary adjustments. For example, the ALG can rewrite the IP address in the SIP layer with its external, routable address before sending it out. It can then keep track of return packets and let them through. It also can be configured to allow specific access to devices behind it.
Manual configuration: Some NAT devices allow manual configuration. In this case, it is possible to put in rules that would allow incoming traffic from the IP proxy's address on the SIP port (5060). However, this requires some technical expertise and isn't very scalable. It also doesn't solve the problem of NAT address as the return IP address. You would not want to ask a telecommuter to implement this solution nor attempt to support it remotely.
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