There are x number of phone lines connected to an 800 number, which
can take x calls simultaneously.  On average, it takes y seconds to
answer one call.  When there are more than x people calling,
additional callers will be put on hold, until someone gets off and a
phone line is available to answer the call.

Let z be the average number of people calling in an hour.   Assume
everyone calls on the second.  Formulate the probability for the
wait-time (t), in terms of x, y, and z.   (The wait-time is the number
of seconds caller be put on hold before the call is answered. )

There is not enough information to answer the question. One needs the
distribution of calls in and the distribution of time to answer a
call. Typically the distribution of calls in is Poisson and the answer
times are exponential. If these assumptions are made, then the
question can be answered.

Thank you for your comments.  This is an automatic phone service
system.  Duration for each call is constant, y.  Assume there are z
incoming calls every hour, and they have equal chance of calling on
any second during that hour.  Thanks again.