The Quintum D3000 doesnt take 2 T1s, it takes one E1 which is the T1 equivalent used in Europe, a T1 is used in North America. A T1 handles 24 voice channels while an E1 handles 30 voice channels. The D2400 takes a T1, you can only only plug 1 PSTN (telephone)Line whether T1 or E1 into a D2400 or D3000
You are correct in thinking that the Quintum D3000 has two E1/T1 trunks and only 30 DSP’s. This obviously only allows a maximum of 30 VoIP calls at any one time.
The reason the Quintum has two digital trunks is that one can be connected to the PBX and the other to the PSTN. The Quintum has a feature (MultiPath Switching) where if the IP network degrades the unit will seemlessly route calls from the PBX port to the PSTN – this allows calls to be made over the existing telephone network in an IP failure. Also you may want to route emergency (999, 911) calls and freephone calls over the normal PSTN network.
The new DX series of gateways from Quintum have multiple E1/T1 trunks with 60 and 120 DSP’s loaded so that they can be used completely for VoIP.