h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Surely the number should be 846 rather than 843, or were the old letters in a significantly different place?

The labelling for dialing Timeline "TIM" should be 846 as suggested.

The letters on GPO phones of the period from introduction of the alphanumberic automatic dialing system are:

1 (no letters)
2 ABC
3 DEF
4 GHI
5 JKL
6 MN
7 PRS
8 TUV
9 WXY
0 O (and later OQ)

Only up to three letters were used on each digit. No provision was made for Z. Local exchange codes were introduced in London, then rolled out across the country. National codes allowed long-distance calling.

City codes were shorter: 021 was "B" for Birmingham 061 was "M" for Manchester, and so on. Uniquely, 01 was London. Then there would be the local exchange number.

Then came the unique subscriber number. So a complete number would be "readable"...

021 744 xxxx was "B" for Birmingham, then "SHI"rley.
021 705 xxxx was "B" for Birmingham, then "SOL"ihull.
0607 xxxxx was "NORthampton"

Not all codes made sense: e.g. a lot of placenames begin with "North" - they cannot all be 0607. NOrthwich got "0606". NON or NOM anyone?

Exchanges were merged, renamed and things got worse... With the inner/outer London split and "PhONEday" introducing 0171/0181, then soon adding more area codes starting 02 (annoyingly, this included the London change to 0207/0208), the squeeze on available numbers means these alpha codes are becoming less meaningful.

Different lettering schemes were used in the US and elsewhere. Amongst other global uniformities, the advent SMS on mobile phones means that the modern telephone is printed with the alphabetic order system...

1 (no letters)
2 ABC
3 DEF
4 GHI
5 JKL
6 MNO
7 PQRS
8 TUV
9 WXYZ
0 (no letters)

Different manufacturers place punctuation in different places.

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Timeline was changed to "123". But despite all of this, dialing "TIM" would still be "846"!!!

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