ex-UK phone engineer here. quickly lurking and noticed the Citrix notable from last bread, they hosts for most of the big mobile networks in UK. all logins to the data backend and tech software run on Citrix servers, as well as all the customer service data - EE, T-Mobile, Orange, BT, Plusnet, Virgin, Asda mobile, I'm sure there's a few other companies that run on the same network too.