set country code as default value in a textbox in c#

Question

Sir I am doing a error prone project at the moment.

I have a comboBox and a textbox.

the combobox stands for coutry list and textbox for phone number.

when i select a country from the combobox list,  i need  that country's code to be enter automatically to the taxtbox.
also the value entered in the textbox doesn't be removed when i enter  phone number in it

please help me on this topic