function formatRFC3966({ number, ext }) {
	if (!number) {
		return ''
	}
	if (number[0] !== '+') {
		throw new Error(`"formatRFC3966()" expects "number" to be in E.164 format.`)
	}
	return `tel:${number}${ext ? ';ext=' + ext : ''}`
}
@param  {object} - `{ ?number, ?extension }`.
@return {string} Phone URI (RFC 3966).