Mobile number validation

Hi,

The first problem should be done with the following:
Limit Checkout Phone field to 10 numbers – Validation

Here is a video about it, you can change the notification text too.

The answer for second question:
Customize Checkout Fields

Thank you @beamkiller it worked ??

and can we add default country code before the phone number field ?

The customise checkout fields doc can apply there too; you can set a default value.