Because different countries have different phone number lengths and different ways of writing phone numbers, there is no single regular expression that would match all countries. Therefore, there are no default restrictions on valid values and by default browsers do no validation. For example, a random value like "abc" will pass validation.
If you need
to enforce a specific syntax for acceptable phone number use the pattern attribute (RegEx validation).
Touch devices with touchpads display the phone keypad, which make it difficult, if not impossible, to enter anything invalid.