Phone Number Validation MVC

Try for simple regular expression for Mobile No

[Required (ErrorMessage="Required")]
[RegularExpression(@"^(\d{10})$", ErrorMessage = "Wrong mobile")]
public string Mobile { get; set; }

You don't have a validator on the page. Add something like this to show the validation message.

@Html.ValidationMessageFor(model => model.PhoneNumber, "", new { @class = "text-danger" })

Along with the above answers Try this for min and max length:

In Model

[StringLength(13, MinimumLength=10)]
public string MobileNo { get; set; }

In view

 <div class="col-md-8">
           @Html.TextBoxFor(m => m.MobileNo, new { @class = "form-control" , type="phone"})
           @Html.ValidationMessageFor(m => m.MobileNo,"Invalid Number")
           @Html.CheckBoxFor(m => m.IsAgreeTerms, new {@checked="checked",style="display:none" })
  </div>

Model

[Required(ErrorMessage = "You must provide a phone number")]
[Display(Name = "Home Phone")]
[DataType(DataType.PhoneNumber)]
[RegularExpression(@"^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$", ErrorMessage = "Not a valid phone number")]
public string PhoneNumber { get; set; }

View:

@Html.LabelFor(model => model.PhoneNumber)
@Html.EditorFor(model => model.PhoneNumber)
@Html.ValidationMessageFor(model => model.PhoneNumber)