How do I forward calls to an outside number from a submenu?

Create an extension that does not point to any phone device or voicemail.  For assistance with creating a "virtual extension", please read How do I add an extension?

Log in to the extension via cp.fonality.com
and activate Call Forwarding for the extension via the "Features" tab

Return to the Admin Panel and click on AutoAnswer --> sub-menus

Select your sub-menu from the list

Within your sub-menu you can add a call sequence to dial the virtual extension you created without voicemail

Whenever PBXtra routes a caller to the sequence in your sub-menu, the call will forward to the number you set within the extension's call forwarding