Mobile Number Validation In SharePoint

Set the onfocusout event for this button as shown below.

<div>Mobile No: <input type="number" id="phonenumber" onfocusout="mobilecheck();"></div>

Create a function representing a link to that button and it gets executed when triggered. 

function mobilecheck() {
    var mobile = document.getElementById("phonenumber").value;
    var pattern = /^[1-9]{1}[0-9]{9}$/;
    //var eml = /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;
    if (pattern.test(mobile)) {
        //alert("Your mobile number : " + mobile);
        return true;
    }
    alert("It is not a valid mobile number. Please input 10 digits only!");
    return false;
}

Go to the site page and start filling in the details.

Here the validation message for Mobile number pops up, if entered wrong.