Our company recently installed an ESI-Tech system through a re-seller. The system seems to work fine and the phones are ok - nothing remarkable. But there is a major flaw in their system when it comes to forwarding calls. I'm a sales person, and when I forward calls to my cell phone they don't just transfer to the phone. They are still tied to the desk phone. Here is how it works and why it is a problem. When the call is forwarded to my phone I answer the call, but then to accept the transferred call I have to press "1" on my phones keypad. THAT is the problem. Calls come to me while driving and in my state it is legal to talk on the phone if it is hands-free. In this case, I cannot take a transferred call! In order to do so, after I answer the call using the button on my steering wheel, I have to pick up my phone, unlock it, access the keypad and then press "1". First off it's really dangerous, secondly it now becomes an illegal action.
This one issue alone likely would have caused me to reject the whole phone system (had I been the one making the decision). If you are looking at this system as an option, be aware that this could be a real problem for you if you have outside sales people.