You didn't say what type of NEC Phone System your running,
However, if you have a Repartee Voice Mail using analog lines the Voice mail is probaly intergrated using MCI (Message Center Interface). This is a serial RS-232 connection between the PBX and the Repartee Voice Mail.
If you require Caller ID for intergration to your new Voice Mail then there will probably need the addition of some hardware. Caller ID station cards, registers and Power supply if on the 2000 for example, not sure about the 2400.
You may lose some functionality using Caller ID there is more information going to the Voice Mail that the calling number.