How can I enforce the E.164 international phone number formatting in a custom Salesforce phone field?

Hi Brook, 

This should work for you:

OR(LEN(Phone) < 8,
LEN(Phone) > 16,
NOT(REGEX(Phone, "\\+?[1-9]\\d{1,14}$")))

Change Phone to whatever your custom field is.

Hope this helps. Steven Trumble |  TrumbleSF.com