Validate phone number

/// <summary>
/// class for validating a phone number in our MVC Model inheriting from the ValidationAttribute. We mark it with the
/// AttributeUsageAttribute <see cref="http://msdn.microsoft.com/en-us/library/tw5zxet9(v=VS.100).aspx"/> To
/// tell the compiler that it can only be used on properties or fields
/// </summary>
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
sealed public class PhoneNumberFormatAttribute : ValidationAttribute
{
    /// <summary>
    /// our format value
    /// </summary>
    public string Format { get; set; }

    public PhoneNumberFormatAttribute(string format)
    {
        this.Format = format;
    }

    /// <summary>
    /// override the IsValid method of the ValidationAttribute class
    /// <see cref="http://msdn.microsoft.com/en-us/library/cc679289.aspx"/>
    /// </summary>
    /// <param name="value">the value we're validating</param>
    /// <returns></returns>
    public override bool IsValid(object value)
    {
        var phone = (String)value;
        bool validationResult = true;

        if (this.Format != null) validationResult = DoesMatchFormat(this.Format, phone);

        return validationResult;
    }

    /// <summary>
    /// method to check if the value provided matches our format
    /// </summary>
    /// <param name="format">the format we're matching too (i.e; 999-999-9999)</param>
    /// <param name="value">the value we're checking</param>
    /// <returns></returns>
    internal bool DoesMatchFormat(string format, string value)
    {
        if (format.Length != value.Trim().Length) return false;

        foreach (string s in format.Split(new char[] { ',' }))
        {
            for (int i = 0; i < s.Length; i++)
            {
                if (s[i] == 'd' && char.IsDigit(value[i]) == false)
                    //return false because we're expecting a number here
                    return false;

                if (s[i] == '-' && value[i] != '-')
                    //return false because we're expecting a character (such as - or ()
                    return false;
            }
        }

        //we made it this far so it's valid
        return true;
    }

    /// <summary>
    /// override the FormatErrorMessage method of the ValidationAttribute class
    /// <see cref="http://msdn.microsoft.com/en-us/library/system.componentmodel.dataannotations.validationattribute.formaterrormessage.aspx"/>
    /// </summary>
    /// <param name="name"></param>
    /// <returns></returns>
    public override string FormatErrorMessage(string name)
    {
        return String.Format(CultureInfo.CurrentCulture, ErrorMessageString, name, this.Format);
    }

}

//Sample usage
[Required(ErrorMessage = "Your phone number is required")]
[PhoneNumber(ErrorMessage = "A valid phone number is required")]
public string PhoneNumber { get; set; }