Yup, unexpected calls tend to make one lose his stance right
haha...I agree.
Still, that method can be used. As a form of rehearsal. we can
always write down some greetings that is said more frequently. Do
mental rehearsal and actual ones by saying out loud, so much so
that it's being internalised.
And when that happens, even when an unexpected call comes, you'll
know how to at least manage the conversation, though you may need
some time to decide on whatever issue that caller wants to discuss
with you.
That, needs more skill. It's a decision making process 
