The ESI S class comes in 2 forms in the ESI world.

The first system made was the S class Gen 1 which went from 4x8x2 to 12x24x6.

This system was fairly limited and the first ones Had a hard drive.

The next S class to come out was the S class generation 2 which had a beefier voicemail and could expand to 16 lines and 32 phones.

The only phones that will work on either S class are the ESI 24 key DFP or the ESI

48 key DFP or 48 key HDFP which had a headset jack.

The newer S class Generation 2 systems also came with an NSP port for programming.