erlang c problem need urgently

A society needs to provide sufficient private trunks between 2 switches to accomodate 90% of call attempts among callers during the busy hour.Assume that there will be 560 callers at random times during that hour, and that call duration averaages 3 minutes.callers are instructed to attempt to call using a trunk; if blocked callers will use a more expensive company-issued calling card.how many trunks will be needed?

in order to eliminate the cost of calling cards , the society in the above problem is considering a new policy that requires blocked callers to redial over and over until they get a trunk.assuming the same number of callers and the same call duration , what is the new level of blockage and about how long is the average total call time incuding both dialing and talking?