Widget Corporation PBX and Voicemail System Design

Problem Statement: The Widget Corporation already has a PBX. But they dont know if they have enough 1) combo trunks, 2) DID trunks, 3) voice mail lines, and if 4) their voice mail computers hard drive is of adequate size. The following represent the stats gathered from Widgets PBX.

Busy hour outbound: 2000 calls per hour with an average holding time of 60 seconds.

Average number of outbound calls per hour on a 24 hour basis is 150, with an average holding time of 40 seconds.

Busy hour inbound to the main number: 800 calls per hour with an average holding time of 2 minutes.

Average number of inbound calls to the main number per hour on a 24 hour basis is 100, with an average call holding time of 30 seconds.

Busy hour inbound direct (DID): 2000 calls per hour with an average holding time of 1 minute.

Average number of direct (DID) calls per hour on a 24 hour basis is 250, with an average call holding time of 60 seconds.

Busy hour internal calls per hour are 400, with an average call holding time of 60 seconds.

Average number of internal calls on a 24 hour basis is 133, with a call holding time of 45 seconds.

Assume that the Widget Corporation requires a Grade of Service of 0.01 (or 1 %) maximum total blocking.

Currently they have 5 combo trunks, 15 DID trunks, 5 voicemail lines, and a voice mail hard drive size of 100mega bytes, and are experiencing some blocking.

Assume that 25% of all DID calls and 25% of all internal calls go to voicemail because the called party is not at their desk.

Assume that the growth rate for Widget Corporation in call volume is 5 % per year.

Solve for the following:

5. Calculate the size of the Voice Mail hard drive, assuming that office workers will be out of the office, not able to check voice mail for at least 7 (24 hour) calendar days projecting 5 years of growth