Do you ever reach full capacity on your telephone equipment? Are there any extensions or trunks/lines that are not currently being used? You may be able to reclaim back unused equipment and save the cost of PBX expansion, etc.
You can run an Unused item report to check all extensions, lines/trunks, etc. over a set period (say 6 months) and then perhaps reclaim them rather than going to the expanse of having to expand your telephone system.