Hi,
Unsure what version of PanOS you are running, but the KB article below should be enough to guide you to where you can disable SIP ALG.
How to Disable SIP ALG - Knowledge Base - Palo Alto Networks
With regards to allowing an external IP access to one of your internal servers, this will likely involve both a Security policy rule and a NAT policy rule. It may not be the best solution from a security perspective though.
I would ask that you provide a little more information before doing this so that we can understand the context of what you are trying to achieve.
Thanks,
Dave
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