Maybe you could try something like this
<form name='MyForm' onsubmit="validatePhone();validateForm();" method="post">
<label for="PhoneN">Phone*:</label>
<input type="number" maxlength="10" name='PhoneN' onkeyup="this.value = this.value.replace(/[^0-9]/, '')" size="16">
<label for="mailN">Your Email*:</label>
<input type="text" name='mailN' size="16">
<button name="submit" type="submit">Submit</button>
</form>