We are given that the number of calls is not more than 10. This means X can take integer values from 0 to 10, i.e., $X \in \{0, 1, 2, ..., 10\}$.
The sum of probabilities for all possible outcomes must be 1.
So, $P(X=0) + P(X=1) + ... + P(X=9) + P(X=10) = 1$.
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