I do agree with alex and rommel. You need to see what is your requirement in terms of how many E1’s and how many extns.(within that digital and analog) with expansion capacity down the line for the next 3 years. ThePBX you choose should be able to cater to this need. Unless you are looking for a very heavy traffic handling capacity i think that you do not have to worry about erlang / switch capacity etc. as almost every pbx comes with a decent traffic handling capability.