Yeah, 1899, 18866, 18185 and their VoIP services often route calls outside the UK and back in again as it's cheaper.
This means mostly when calling a BT landline it will display as "International" even if the person is calling from next door, as the CLI is often flagged as "0044......." or "44....." BT annoyingly most the time change this to "International" if it can't be 100% sure the of the source / the number is genuine.
Occasionally BT allows the number to be displayed, and why sometimes when you get calls from the USA, France, Germany, Spain etc the number is displayed.
You don't get any of this problem with mobile phones, when they see the number as +44 or 0044 it recognises you are in the UK and formats the number correctly and will also display international numbers.
This is why I've said on other threads, if a call comes up as "International" on your landline then you shouldn't just ignore it, as it could be friends, family, important business calling, but have chosen to use 1899.
Who wouldn't use 1899 when they only charge 3 pence per call anytime of the day.
I dumped my landline last year and just use VoIP, the great thing about VoIP other than loads of free services, such as call waiting, ACR, 3-way calling and caller display is that it will display all international callers numbers if not withheld and not just mask them as International as BT mostly do.
It is only a tiny %-age of calls I receive that aren't routed via 1899 or one of their sister companies.