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.