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Community Member

How many T1s


Are there any guidelines for determining how many phone lines to order? Let's say we have 80 users. Right now, we have 80 analog phone lines. In a VoIP world, to match that, I'd need to order 4 T1s if I want to be able to support everyone on the phone at once.

But in a typical office environment, not everyone is on the phone at once. Are there any formulas/guidelines telco's use to help size how many phone lines you should order?

I realize of course it all depends on the profile of your users. Call centers might need 100% capacity. But in a standard office, I'm guessing more like 60%? Anybody have any examples they've used?


Re: How many T1s

Here are some of the questions

1. How many simulataneous calls are made in your environment. You should do an analysis when you do capacity planning. Each T1 can facilitate 23 simulataneous calls

2. Do you need redundancy for your T1

3.How many sites do you have, and how are your sites connected.

There is an assesment tool available on cisco website you can use for VoIP readiness, or you can contact cisco for more help.

Community Member

Re: How many T1s

Thanks for the response.

1. I'm afraid we don't know. We just have standard analog service from our teleco, and I'm not sure they provide that level of reporting. I'll look into it.

2. No, we do not.

3. Just our one site.

Side question, I know a T1 could carry 24 lines. If it can support 23 calls, what is the extra "line" for?

Hall of Fame Super Red

Re: How many T1s

Hi Shane,

Just to add a note to the good info from my friend Rajesh :)

The old telco rule is called the "1 in 10" Trunking rule. 1 Trunk/Channel per 10 users.

You can always engage your service provider to do a Traffic Study that will also include busyhour averages in your current setup. It is probably good to be over-trunked rather than under-trunked, especially with an Enterprise environment. So in your case this would equal 2 PRI's if you wanted Failover support as nicely noted by Rajesh.

1 Trunk (in your case channel) per every 10 users, so

100 users = 10 channels

200 users = 20 channels

You can also use RTMT to monitor your PRI's and send you email alerts when defined thresholds have been exceeded.

As you can see this is not an exact science for example;

For our Student Residence we use 5 PRI's for 1000 student phones.

For our Staff we use 4 PRI's for well over 2200 phones.

Hope this helps!


Community Member

Re: How many T1s

VERY helpful, thanks for the examples!

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