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Hey guys! Not too positive about the relevance of this question to Server Fault, but I'll let you guys poke around at it anyway. :)

I'm trying to figure out what an accepted ratio of endpoints to concurrent calls on the "average" office PBX. I'm defining concurrent calls as any call being placed, including internal calls. I'm not going to be considering call centers into this ratio, I'll be looking at those differently, anyway.

To give you guys a little history behind the question: I need this ratio to be able to figure out decent specs for a virtual PBX service I'm going to be rolling out within the upcoming months. The ratio will determine the number of trunks needed per PBX system.

-- Logan

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I would say that it's in a wide range that depends heavily on the type of business it is and how they conduct their activities. – Dennis Williamson Nov 22 '10 at 16:34
@Dennis Williamson: That's why I said "average." :) We're not talking about a sales team or a call center, this is just a typical office of people either doing data entry or analysis or what have you. The reason why I don't know the information myself is I've never worked with the "average" office. I come from the healthcare industry where I managed an Avaya PBX with 10,000 endpoints. At the most, there was 70 or 80 concurrent calls. But, on average, there was 800k calls per day. :\ – Logan Bibby Nov 22 '10 at 20:04
You can lop off the outliers and there's still a wide range. By the way, those numbers come out to about 8.6 seconds per call for a 24 hour work day. – Dennis Williamson Nov 22 '10 at 20:24

2 Answers 2

You're going about this backwards. You need to figure out the load that is needed to sustain operations, and build out the PBX based on those numbers. Given today's processor power you're going to be more limited by the bandwidth available to you than anything else.

you can try using something like this to figure it out.

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Thanks for that link! That tells me the maximum recommended limit on calls a single PBX system can make, which is very helpful indeed. :) The reason I want the "average" office's concurrent calls is for budgeting and determining how much to charge, what the box's (which will be virtual) specs should be at a minimum, etc. Thanks again! :) – Logan Bibby Nov 22 '10 at 20:08
@Logan for telecom systems you need to engineer for peak not average otherwise you will be dropping calls at peak times. – Zypher Nov 22 '10 at 20:21
I'm going to be using virtual boxes with practically unlimited resources for each box. I think I've found a decent way of estimating. Once I'm able to get my own data coming in, I'll be able to crunch the numbers and figure out what my typical client will need. – Logan Bibby Nov 24 '10 at 21:34
@Logan be careful thinking you have unlimited resources, you don't. You also introduce a new set of issues. Especially Timing issues (Very important with telecom) and CPU contention issues - especially if you are hosting multiple VMs on the same hardware. – Zypher Nov 24 '10 at 23:06

Your math doesn't look good the 880k calls per day must have been for a couple of seconds to work with 70 or 80 concurrent calls, although I could believe you had at most 70 or 80 concurrent external calls. Auto-dialers for paging and credit-card validation also through counts off.

The figures I used for data connection calculations was 8 users per connection. This also worked well for our PBX. To get a good estimate figure out what hour(s) the phone system will be busy, then figure out how many minutes of that hour the typical user will be on the phone. 8 to 1 gives you 7.5 minutes per hour. You will need some extra capacity usage will not be constant during the hour.

All bets are off if everyone starts work at the same time, and immediately start using the phone to return calls and check voice-mail.

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I'm not sure how much of our traffic was auto-dialers and credit card validation, most of that data was privileged for our director. I do know we had several internal call centers and a switchboard department that took in atleast 1,000 calls per day... Crazy stuff! Anyway, 8 to 1 sounds like a decent calculation! I was actually thinking more along the lines of 3 to 1 for us since our trunks are pooled across the entire system. :) Thanks! – Logan Bibby Nov 24 '10 at 21:36

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