Telephone Systems Handsets
Estimate the number of business telephone system handsets you expect will be needed.  The number of telephone system handsets is typically determined by the number of employees that have, or should have, their own phone. You should also include any reception or common areas (i.e., meeting rooms, lobby, etc.)

Business Telephone Systems Lines (smaller systems)
Telephone system lines are the total number of outside lines needed by the company.  The number of phone lines is equal to the maximum number of simultaneous inbound and outbound calls the company can make on its phone system. For example, a telemarketing company may need 40 telephone lines to service 60 handsets, while a manufacturing company may only need 15 telephone lines to service the same 60 handsets.

Business Telephone Systems Trunks (Larger Systems)
Telephone system trunks typically refer to the total number of connections between your business telephone system and your phone carrier. Tie trunks are used to connect the phone systems between multiple offices.  For most small to medium US and Canadian businesses this is called a Trunk Level 1 or standard t1 line - Europeans use an E1 with slightly more capacity. For smaller companies that still need digital access for their business phone system, or the Internet, may use xDSL or a fractional t1 line. While, larger companies with heavy communications needs may step up to a ds3 or oc3 trunk.

Business Telephone Systems Ports (PBX)
The maximum number of simultaneous telephone service connections that you will need to connect to your business telephone system. Telephone services include outside lines, inside extensions, voice mail, etc. the system will need to accommodate simultaneously.