Call Centre/Erlang Formula Problems

Good morning,
I have a problem and I'm afraid that I've pretty much come to the limit of my Excel skills!

I'm trying to create a very basic scheduleing/intraday spreadsheet for the call centre where I work.
I've got to the point where I know what the average call volume is per 15 minute interval, but now want a formula that is able to take that number and give me the number of people required to answer those calls (SLA being 85% in 15 seconds, with a 240 second call time).

I have had a look at the free Erlang calculators that are out there, but none are really fit for my purposes, and have Googled for the formula itself, frankly it's frazzled my brain!

Is the above even possible, or should I just keep shouting at my boss that we need a Work Force Management system...?

Many thanks in advance.

Apologies for the delay in replying, and thanks, I'll have a look at the link.