PhoneCall
Texting explain how the Phone and sms objects work.
w.r.t. Phone
PhoneCallEnded( status , phoneNumber )Event indicating that a phone call has ended. The status
can be any of:
-
1
: Incoming call was missed or rejected
-
2
: Incoming call was answered and hung up
-
3
: Outgoing call was hung up.
PhoneCallStarted( status , phoneNumber ) Event indicating that a phone call has started. The status
can be any of:
-
1
: Incoming call is ringing
-
2
: Outgoing call is dialled
You can experiment . 0 is NOT an option available; but you can try it, not all options are necessarily documented.
You do not 'Clear' TextBox1 Waiting message, possibly you should use the PhoneCallEnded with a status of 1 or 1 and 2 to clear the content of textBox1?. Also, you might add, at least temporarily another textbox so one holds the phone number and the other the status message?
Your app, you got to experiment. Eventually you should find a solution and can let us know what happens and works. 
note that while that is true in your case, it might not be permitted when using both the Texting and Phone call components. I don't know, so you should consider that possibility.