Comment 1 for bug 1404301

I would avoid disabling the call button. I think users should still be able to enter USSD codes during a call, so the call button needs to be enabled all the time. Also, other protocols like sip might allow this use case (call another number while the current call isn't fully active).
There is another use case that we currently don't support but also needs the call button enabled, which is pressing the call button when no number was entered to fill the entry with the last called number.