There are lots of great systems out there now such as the IP Office, Cisco Call Manager, Panasonic, Toshiba and Shoretel.
Well I am partial to the New UCx system from E-MetroTel which is based out of Plano Texas. The system is designed by former Nortel guys who developed a switch called the UCx that is designed to continue supporting legacy Nortel equipment (especially BCM and Norstar) while adding new phones to it.
They can do exactly what you want and you don't have to replace the phones or cabling, and most of the feature codes stay the same. Then at the same time, you can add IP phones of your choosing such as the Cisco 79xx series, or the SNOM phones, or my favorite, the Nortel Unistim IP sets such as the i2000, 11xx and 12xx sets. The system supports SIP trunking, PRI/BRI and analog trunking.
Contact them, I know they have had a lot of call center/call queuing customers utilizing their M and T series sets, so they may be able to accommodate what you want at a reasonable price.
So the big advantages? You will not have to rip and replace, only replace the equipment in your phone room. You won't have to retrain your employees on every new feature.
"Keep the Peace, Use RLS"