Hi James,
I use EE and Vodafone both have issues depending on the summit. EE is my monthly contract and loses network on about 5-10% of the summits. Vodafone is a PAYG (no data) and gets a text signal on about 50% of the summits that EE doesn’t.
Your location on the summit also makes a difference for example on Dale Head, Robinson and High Raise recently operating my location didn’t get a network signal on either until I stood and moved. On EE often my data connection is either E or at best H+.
Good thing about Vodafone as a backup is the topup doesn’t expire after a month as it can do on other network PAYG.
Neil G0WPO