I'm guessing they're well covered by 100% never guaranteed clauses and I suppose many have WiFi access. What I would say is better comes to say there was a problem, what was affected and when fixed would be better. Easy for me, I never get stressed but not so easy for business users and those with family issues I guess.
Yes, that has been happening with me - a text sent to me is not leaving the sender's phone (network error) but is landing on my phone several times a minute.
Oh the joys of fixing an aging infrastructure... 
@Ferretti wrote:All these 'you can make calls another way' mails from self satisfied customers is all well and good, albeit impractical in my case, but it doesn't tell me how I'm supposed to get around the fact I am also unable to receive calls, which will doubtless annoy my customers. This in turn may very well lose me money, both today and in the long term, so please excuse me being a little upset.
I needed to contact a person this morning ~ we are both on O2 (we didn't know this) but
we resolved it this lunch-time, smiles and goodwill.
It is my experience that most Customers are very responsible and will completely understand.
Maybe your customers are on O2 or maybe they will hear of the problems.
It is very often that I call a Trades-person and can't get a reply ~ I don't chuck the number in the bin,
I make a further call or enquiry.
I hope everything will return to normal soon.
Glad to report one positive out of this... Our office vending machine is unable to take credit cards as it's unable to make a mobile data connection to validate them. My waistline thanks the good people at Ericsson for caring about it 
