It's short for "cellular network", the American term for the way the system works.
Yes, they are called "mobile" in the UK, but they are not mobile!
It only shows whoever dreamt up the term was unable to differentiate between mobile and portable, and even if anyone did point it out, you don't expect an IT or telecommunications company and its advertisers to take any notice.
If the standard of English in BT's advertising and on parts of its Internet service are a guide, I am not at all surprised at such things.
My god....I had a feeling that my phone was mobile when I found it in my pocket while I was out and about in the middle of nowhere and it still worked...lol
I do call it from Don's phone, a lot. I misplace it A Lot. When I am talking about it, I address it as I-nstine because it's an iphone and can do many more things than I know how to do on it, with it, for it, or because of it.
Americans do... and Britons call 'em "mobile" only thanks to some barely-literate advertiser. (They are portable, not mobile.)
I call mine "portable", and a couple of times have had had people say, "Don't you mean 'mobile'?", to which I say, "Well, it doesn't have wheel and a motor, does it!" They usually end up looking flummoxed.
Until it relaxed the rule a little, my employer had a notice in the Reception, saying "Portable Telephones Are To Be Switched Off". Hooray! Well you'd expect correct terms in a company specialising in sophisticated electronic engineering.