Asp.net web service read custom header

Question

cant read http custome header "MSISDN" from my web service ... same code works on regular asp.net page. Can anyone shade a light on that subject ?

    public CellulupResponse parseRequest(string requestString)
{
    CellulupResponse response = new CellulupResponse();

    UserIDType userType = UserIDType.MSISDN;

    // Get The HttpRequest Object for getting 
    // the msidn for the requested user.
    HttpRequest Request = this.Context.Request;

    string userID = Request.Headers["MSISDN"];
    string userAgent = Request.Headers["User-Agent"];

    if (String.IsNullOrEmpty(userID))
    {
        userID = Request["X-Wap-MSISDN"];
    }

    if (userID != null)
    {
        if (userID.Length > 20)
        {
            userType = UserIDType.XID;
        }
    }
    else
    {
        userID = "";
    }

    Charge CellulupCharge = new Charge();
    response = CellulupCharge.parseRequest(requestString, userID, userType, userAgent);

    return response;
}