SIP Trunk External IP


I’m using Skype for Business server and have an issue with NAT in the fact that my SIP provider are authenticating me by my external IP address. Problem is, Skype for Business is providing my internal IP address. I do have a DrayTek that supports SIP ALG but 1) Ive read bad things about SIP ALG and 2) It doesn’t work anyway.

Can I configure it so that my Skype for Business config uses the freepbx as its gateway and then connect the freepbx to my SIP provider and that present my external IP address?

If so, how would I do this?

Thanks in advance.

Can anyone help with this at all?