Adding a reference to a program in C#

Program problem: collect data from the user then output data in the list box. Data that needs to be collect is the model of the phone, brand, and price.

My code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;

namespace Cell_Phone_Inventory
{
    public partial class Form1 : Form
    {
        List<CellPhone> phoneList = new List<CellPhone>();

        public Form1()
        {
            InitializeComponent();
        }
        private void GetPhoneData(Cellphone phone)
        {
            decimal price;
            phone.Brand = brandTextBox.Text;
            phone.Model = modelTextBox.Text;

            if (decimal.TryParse(priceTextBox.Text, out price))
            {
                phone.Price = price;
            }
            else
            {
                MessageBox.Show("Invalid price");
            }
        }
        private void addPhoneButton_Click(object sender, EventArgs e)
        {
            CellPhone myPhone = new CellPhone();
            GetPhoneData(myPhone);
            phoneList.Add(myPhone);
            phoneListBox.Items.Add(myPhone.Brand + " " + myPhone.Model);

            brandTextBox.Clear();
            modelTextBox.Clear();
            priceTextBox.Clear();

            brandTextBox.Focus();

        }
        private void phoneListBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            int index = phoneListBox.SelectedIndex;
            MessageBox.Show(phoneList[index].Price.ToString("c"));
        }
        private void exitButton_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}

It all seems to be correct, however, I am getting an error code of CS0246 and CS0108 under all the "CellPhone" words.