You make the mistake people have always done. That figure is EBITDA.
Most of that money will go in tax, and investment to keep the network running.
It's like expecting a taxi driver to set his fares according to his operating (petrol, servicing) costs alone. It'll work for a while, until his car wears out and he can't replace it.
BT have thousands of exchanges, which like any modern electronics have a life of 15-20 years before they are dead or obsolete (unlike old mechanical stuff that could last 30+). It's been a while since I was close to that industry, but assume 5000 exchanges. 20 years life, so that means replacing 250/year. That's one per working day. At several £m a time you'll burn £5bn very quickly.
It's a huge company, it has huge income and huge bills. By most measures they made very little money for their size, hence the hammering the shares took.
Sure, you can do phone networks on the cheap. Like my ISP, which is averaging 50+ hours of outage per month at the moment. Doing it right costs money.