How it is ?

i am working with DTMF for dialling.in button click i have written

char digit1 = Convert.ToChar(((Button)sender).Text[0]);

iax.sendDTMF(digit1);

txtCallTo.Text += Convert.ToString(digit1); here it is working i am adding the digits to the textbox txtcallto as

txtCallTo.Text += Convert.ToString(digit1); so it is working But i want to call the same function in the textbox_textchanged event.there i am not able to add digits to textbox as i did in button click i.e. txtCallTo.Text += Convert.ToString(digit1);
so how can i add to the textbox