Answer
$10\ callers$
Work Step by Step
We know that
$\beta=10log(N)+10log(\frac{I}{I_{\circ}})$
We plug in the known values to obtain:
$120=10log(N)+110$
$\implies 10=10log(N)$
This can be written as
$N=10^{\frac{10}{10}}=10\ callers$
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