Normalises a phone number to E.164.

Its own module with its own test table, because this function decides whether a message reaches a person or goes quietly nowhere. A number typed 070-123 45 67 in a form, 0046701234567 in an import and +46701234567 by an API has to become the same string before it leaves the app.

The rules

"070-123 45 67"    → "+46701234567"   # leading 0 → country code
"0046 70 1234567"  → "+46701234567"   # 00 → +
"+46701234567"     → "+46701234567"   # already E.164
"0701234567"       → "+46701234567"

"123"              → :error           # too short
"08-12"            → :error
nil                → :error

The country code

:default_country_code decides what a leading 0 becomes. It defaults to "46":

config :ash_dispatch, :default_country_code, "47"

A number that already carries + or 00 is left alone — the country code applies only to the national format, where the zero IS the trunk prefix being replaced.