It's a ridiculous telephone when you look at it, however it is the hotline to Number 1, and vice versa to those who are second only to 1, that's Number 2.
At the beginning of the episode ‘A B and C,’ Number 2 has a nervous look on his face as the telephone begins to bleep, he picks it up
“Number Two here. Yes sir, I am doing my best. He's very difficult. I know it's important.......... He's no ordinary person sir, but if I had a free hand........ I know sir yes I'm not indispensable.”
Later, after Number 6 has paid Number 2 a visit, he is again put under pressure by Number 1 via the hotline. “Yes sir, within two days. You have my word....... Yes sir, I realise my future is at stake. Two days, I guarantee.”
Yet despite his word and guarantee, it all goes pear shaped for Number 2, and it is as the oversized red telephone begins to bleep in the laboratory, he knows what is coming. In fact Number 2 had half expected the telephone to start to bleep after the failure with 'B', but now with the complete failure to discover the reason behind Number 6's resignation Number 2's fate is sealed.
Number 2 of ‘Hammer into Anvil’ is another Number 2 to have access to the red telephone, well not all Number 2's use this peculiarly over-sized red, curved telephone you know. Although it is seen on the desk of certain Number 2’s in later episodes. But it is only actors Colin Gordon and Patrick Cargill who are forced to use this prop, although Gordon does get away with not having to do so in ‘The General.’ Anyway it is down this telephone that Number 2 gives Number 1 all his assurances, that he doesn't need any help, that he can manage. And it is a second Number 2 who gets the chop by telephone, even though he has reported himself as being a breakdown in control, and in need of being replaced.
Actually, this telephone sitting on Number 2's desk must be a constant reminder of Number 1's ever presence, being just a phone call away. I mean no-one is free of observation. Take Number 2 for example. There he was, having learned of his failure to Number 6's success, that almost instantly that oversized red telephone began to bleep. So Number1 must surely have been watching to have learned of Number 2's failure so quickly.
Be seeing you