Calls cutting off to specific numbers

Caz3121

been tasked with helping parents with an issue and would be good to know which direction to head to for help.
They moved house 6 months ago and landline and broadband are via NowTV/Sky. There are two phone numbers (friends of mums) where any calls made either of these numbers cut off after 56/58 seconds (resulting in recipient calling them back) Does not appear to happen with any other numbers and is becoming a bit of a frustration.
I have seen this thread https://forums.moneysavingexpert.com/discussion/6450567/vodafone-landline-cuts-off-every-29-minutes but different times.
Not sure whether to go to NowTV/Sky or Openreach first for resolution (if there is one!)

born_again

You go to the suppler, so NowTv/Sky.
You have no contract with openreach.

iniltous

It’s not got anything to do with Openreach .