http://www.csgnetwork.com/directerlangprob.html

Sounds like you're in over your head :)

Let's do math.

1 Erlang = 1 call up for a whole

Presuming you were open for 8 hours and had 1200 calls over those 8 hours, you'd have 150 calls per hour.

If those calls were of 3 minute duration, you'd be at 450 minutes of traffic per hour. Divide by 60 and you're at 7.5 erlang.

7.5 lines would cover that amount fine if the public were kind enough to call you on smoothed out patterns which they obviously will not.

Still, 1 PRI is fine based on the calculations I've understood.

Now, if you answer for the Home Shopping Channel or the Psychic Hotline where a toll free number shows up on the TV for a minute and everyone gets told to call it RIGHT NOW, then you're not exactly smooth. You might expect 500 calls in the next 10 minutes and then peace and quiet for another 10 minutes and then another 500 calls, etc

The probability of your call being blocked is called Grade of Service. https://en.wikipedia.org/wiki/Grade_of_service

In theory, nothing stops the whole world from calling you at once and blocking up your trunks and you giving busy signals. So even with a million trunks, there's a one in a bajillion chance that a million people felt like calling you right now and caller 1,000,001 gets a busy signal. Based on your "known volumes" that would be statistically insignificant to the point where you'd be more likely to win the lottery a few weeks in a row.

Are you sure you need to cover just 2200-3600 mins/day?