First you have to find out how your holding time is distributed. normally this should be negative exponentially. Then you must determine the average holding time. After that you can calculate the probability of x call arrivals during unit time assuming that the arrivals do not overlap and that the probability of x arrivals in time t equals.
This can be done using the Poisson formula.
After you have calculated the Poisson distribution, just sum up the probabilities of having n arrivals until you come to (for example) 0.99. Then the probability that (due to too many arrivals at the same time) the system utilization exceeds 100% is 1%. That means, if you have summed up the probabilities from 0 arrivals up to (for example) 25 arrivals, that the probability of upcoming arrivals not being served is 1% after already having 25 arrivals.
You can also calculate the blocking probability using one of Erlang equations (dependet from the type of VoIP system you use).
To calculate the bandwidth, you must consider, that not all bandwidth can be used. (e.g. today’s switched Ethernet allows a maximum of 90% bandwidth utilization)