If the phone was on a version 6 system previously and is a 5330e/5340e series phone, you cannot take it back down to a version 4 system and use it... When the e series phones are shipped from Mitel, they have a generic firmware which will run on the 3300 or 5000 in a 5330/5340 (non-e) fallback mode so to speak, you will notice if you take a new one out of the box and put it on a older system it fails to download the firmware via TFTP... Once the phone has been connected to a system that supports it, it pulls the matching firmware via TFTP and the original firmware is overwritten, making it unusable on older systems that don't support the e series phones directly.