RFC 2806 defines the "tel" URL scheme - is this what you are looking for?
Yeah perfect! Thanks Daniel! I found RFC 3966 is the newer version of 2806,
and it goes OTT for the purposes for Sieve, but if we pick selected bits of the
BNF we get:
telephone-subscriber = global-number / local-number
global-number = global-number-digits *par
local-number = [I'd suggest we'd not allow this]
par = [I'd suggest we'd not allow this]
global-number-digits = "+" *phonedigit DIGIT *phonedigit
phonedigit = DIGIT / [ visual-separator ]
visual-separator = "-" / "." / "(" / ")"
Then it's a fairly trivial case of binning the visual-separators and you should
be left with a string of numbers prefixed with a +. :o)