Hi Partner,
You can achieve it easily through Editing phone field in HTML editor of marketing form:
(1)Check the HTML of the marketing form, the phone type is 'tel':

Then i add pattern attribute to the <input> tag, which specifies a regular expression:
Note: The pattern attribute works with the following input types: text, date, search, url, tel, email, and password.'
My example: placeholder="123-45-678" pattern="[0-9]{3}-[0-9]{2}-[0-9]{3}"

When i try to submit form with wrong format phone number, message will appear:

Correct format:


(2) Or you can change type to 'number', then the phone field will only allow numeric input.

Test:

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For complex validation, we can achieve it with form API and custom logic.
https://docs.microsoft.com/en-us/dynamics365/marketing/developer/marketing-form-client-side-extensibility#examples