Holborn exchange had 9400 subscribers (lines) and had 222 directors, each of which would on average handle 72 calls in the busy hour (BH) and be held for 20 seconds.
By 1950 the London director system had 75 exchanges within a 5mi radius of Oxford Circus and a further 65 in the 5-10 miles (8-16 km) belt; and in Greater London i.e. within 20mi of Oxford Circus there were 237 exchanges altogether [REF].