Use this Identification Guide for the Nortel Norstar T-Series line to identify the telephone on your desk. This will help you purchase the correct replacement parts the first time!
The T7100 desk set is the smallest with no programmable buttons on the right side.
The T7208 has 8 programmable buttons on the right side and the T7316 and T7316 E both have 16 programmable buttons plus an extra 8 feature buttons.
The easiest way to tell is you have a T7316E is to look for the third button under your volume bar. See the red oblong shape in the illustration below.