function isPhoneContextValid(phoneContext) {
	if (phoneContext === null) {
		return true
	}

	if (phoneContext.length === 0) {
		return false
	}

	// Does phone-context value match pattern of global-number-digits or domainname.
	return RFC3966_GLOBAL_NUMBER_DIGITS_PATTERN_.test(phoneContext) ||
		RFC3966_DOMAINNAME_PATTERN_.test(phoneContext)
}
Returns whether the value of phoneContext follows the syntax defined in RFC3966.

@param {string|null} phoneContext
@return {boolean}