nighthawk x6 r8000 only allow incoming from

Hi All

I have a Nighthawk X6 R8000 (firmware V1.0.4.68_10.1.75)

I have a hosted VOIP telephone and keep getting "ghost" calls, The provider has told me that I need to add a rule to the router to only allow ports 5060-5070 from the IP: 3.9.55.138

Anyone know how to do it please?

Many Thanks

Steve