Static IP Address

Easy you would be paying for a block of 8 IPs (5 usable) for $30 per month. Your AT&T Gateway needs to be configured based on the instructions from AT&T showing your Public IPs.

Dave

The person can use your existing public IP.   It won’t change on Att broadband connection. 

Is it necessary for your personal router to run in IP passthrough mode?   If yes, you may want to confirm the settings in the gateway are correct.  Post screenshots here.  

It depends on what you mean by "static address?"  A Public Static address (as dave006 says), you pay for.  This would be a block of IPs that people outside your network could use to reliably find your network, because they do not change (at least without a lot of notice).

In contrast, the single Public IP you have now is technically called a Dynamic address, meaning AT&T can change it whenever without any warning.  In practice, however, AT&T does not change your Dynamic Public IP very often.  Mine has currently remained unchanged for 27 months, it changed when I moved from VDSL2 to Fiber, and before that it was unchanged at least twice as long as that.  I think you're probably find using your Public Dynamic IP.  IP Passthrough is a feature that deals with your Public Dynamic IP, not so much with a Static Block.

Now, you might want an internal private IP to remain the same and not change whenever it is powered off for a long time, etc.  That could be done either by statically assigning it a private IP and/or making the dynamic assignment sticky (which is done on the Home Network > IP Allocation tab by clicking Allocate on the button with the device and IP you want to make sticky).

Do you have a SIP phone or soft phone or what exactly is your problem with VOIP?

Have you tried changing your SIP ALG setting on the Firewall > Advanced tab (try whatever it isn't now to see if that fixes your issue)?

Thanks to each of you for responding. All very informative but mostly way above my pay grade.  I purchased the block of static IPs and passed the info on to the VoIP tech support guy who requested it.  Going to let him take it from there. Thanks again. 

Good luck with that.  If paying for a block of static IPs fixes a VoIP problem, that will be a first here.