We're trying to work out what a safe number is for our call centre - BS9999 / ADB Vol 2 show 6m / person for Offices (as a guide)...

But, we have introduced smaller desks (1200mm x 800mm) as opposed to the original desks in the office space.

Is there a specific guide for calculating the occupancy capacity for 'call centre' environments?? 

As always, any guidance always very much appreciated! 

Looking at all the various questions on here for different premises it looks like you need to employ the services of an external competent person.

Call centres usually have a higher density than offices and in older buildings, constructed based on office use and before call centres were a thing, will usually only have means of escape based on the lower office density and so a change of use results in overloading the capacity of the escape routes, both vertical & horizontal.

I would first calculate the maximum capacity of your existing escape routes as this often is where the limiting factor comes in.