This is from an email discussing the issue from Dr David Whitaker:
Most modern switchboards are now computerised so can accept any number
– If 5 or 6 digits used can usually still accept 4 digit 2222 (our own hospital in Manchester does this, we had 2222 from 2004 but then got a new 5 digit switchboard in 2011 but it could still accept 4 digit 2222 so we kept it at 2222.
– If 4 digits not possible then just use 222 or 22222
Older switchboards and smaller hospitals with fewer extensions may only be able to use 3 digits so to involve them and let those with a safety culture take part using a 3 digit 222 is good and anyway if a new member of staff form elsewhere dials 2222 it will still activate the emergency response –so quite safe. Low / middle income countries may use 3 digit systems but good to have them involved in standardisation.
If for some reason hospitals can only use 5 digits 22222 well it is better and more memorable than something different like 69345 0r 36042.
As regards the history the original suggestion by Phil Lawler et al in 1993 was 222 as it was commonest in a UK regional survey in 1993. By 2004 the NPSA survey showed 2222 was commonest as I guess by then most NHS hospitals had got 4 digit switchboards so that is why 2222 was recommended/ chosen. As modern technology can still keep 4 digits it remains the standard number and “double 2 double 2” is memorable.