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.
I hope this helps, if you need any further assistance please feel free to contact me at: firstname.lastname@example.org