If this is for inbound calls, they may be requesting that you open port 5060 by forwarding it to your dialer.
If this is for outbound, I'd have to check the deeper asterisk core for how to lie to the server and put port 5060 in the outbound message instead of the true rtp port assigned by your router. Note that if you pass port 5060 there, instead of the rtp port assigned by your router, the reply will likely arrive on port 5060 and be dropped by your router UNLESS you forward port 5060 to your dialer. FYI.
My advice would be to ask them what the configuration parameter is for asterisk that they want configured. Seems a little fishy, and possible that they are simply not communicating what they really need properly. Since it's not "standard" they should have an example configuration entry available to give to clients using asterisk (unless they are truly tiny and have no asterisk clients?).
In any event, your best bet for a response to this may be on an asterisk forum, as this Vicidial forum is tiny by comparison. Asterisk is HUGE and has many forums with detailed experts in the idiosyncratic bits inside of Asterisk