function diallable_chars(formatted_number) {
	let result = ''

	let i = 0
	while (i < formatted_number.length) {
		const character = formatted_number[i]
		if (DIALLABLE_CHARACTERS[character]) {
			result += character
		}
		i++
	}

	return result
}
A map that contains characters that are essential when dialling. That means
any of the characters in this map must not be removed from a number when
dialling, otherwise the call will not reach the intended destination.