how is it possible

I have used convert.ToChar(Textbox.text[0]);
here it is raising error if i dont specify the index[0].
actually i want to do like this conversion
digit2 = Convert.ToChar(((Button)sender).Text[0]);
            iax.sendDTMF(digit2);
            txtCallTo.Text += Convert.ToString(digit2);
this code i have written in button event so there is no problem,now i want to write this for textbox.how?