Glossary
What is E.164?
E.164 is the international phone number format: a plus sign, the country code and the number with no spaces or leading zero, as in +447700900123.
UK numbers get written all sorts of ways: 07700 900123, (07700) 900 123, +44 7700 900123, 0044 7700900123. E.164 boils them down to one form by dropping the leading 0 and adding +44, which gives +447700900123. A system that stores every number that way can match a reply to the right contact and spot the same person entered twice. (That example number comes from a range Ofcom keeps for TV and film, so it doesn't belong to anyone.)
A sports club in Exeter importing a spreadsheet of members' numbers, typed in by different volunteers over the years, ends up with one consistent format, which makes duplicates easy to spot.
CaptiFi Connect accepts UK numbers in any of the usual formats and stores them in E.164. Mobile numbers from the UK, Guernsey, Jersey and the Isle of Man are accepted; landlines and numbers from other countries are refused with a reason and aren't charged.