How to create a phone validation (regex)

Hi,
i’m trying to create a validation for phone numbers.
I would like that using a text field I can set that only numbers and a + before them (if someone write that) can be accepted (for example +3902445566 or 003902445566 both valid).
I did that to try:
-create a new text field and write the regex code in the field ‘regex validation’ then I write the regex failed validate in the other field).

But after that also when I write a regular phone number I always have the same regex failed validate message. I think it means t’not the correct code or not the correct method.

Could you provide an example that I can try on my site?

Thanks

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