Hi JustLearning,
I agree with Ravi that the most appropriate method is to use JavaScript and regular expression to limit the text.
If you recreate the Mobile Phone field as a whole number field, it is not suitable for 11 digits.
Because the maximum value of whole number field is 2,147,483,647, does not match the range of 11 digit phone numbers.(0~99,999,999,999)

And if the mobile phone starts with a few 0, the first few 0 will be deleted. E.g. 05101234567->5101234567
So the best way is to use JavaScript and regular expression.
Hope this helps.
Best Regards,
Lu Hao