If a system is already in existence, it can be monitored to help decide which features are needed and which aren't. Many smaller systems are limited to a maximum of 20 or 50 extensions, with maybe eight incoming trunks. Some of these systems use phones that are incompatible with larger systems from the same manufacturer. If the business plans call for high growth in one location, these systems should be avoided. Considering that every phone and component in the system practically never fails all at once, it is likely that the system chosen today will be around until it's outgrown.
Many systems allow themselves to be tied together, especially over distances.
Some systems limit the number of outbound trunks that may be tied up in a conference call, while still allowing many internal extensions to participate. Some systems can only use as trunks standard business lines with no DID capability. Others may have cards that can be used that allow incoming trunks to be ISDN or T1 trunks. These high-capacity lines allow 24 simultaneous trunks, and are cheaper to get than the equivalent 24 individual wire pairs. ISDN trunking in particular is highly desirable as it allows a much higher quality call, as well as the ability to pass CallerID information down DID trunks.