First: who answers when it breaks? A phone system is a service relationship, not a gadget. If the answer is a national support queue, that is the product you are really buying.
Second: what happens to your numbers? Any decent provider ports your existing numbers free and lets you take them when you leave. If either half of that is missing, walk.
Third: what does adding a person cost? Per-seat cloud pricing means growth is a config change. Old-style systems mean a truck roll and a quote.
Features are mostly identical across vendors now. Auto attendant, voicemail to email, mobile apps, all standard. Buy the relationship and the pricing model, not the feature list.