I can only speak from the Avaya side of the house, as that is where my experience is. That being said, here is my best attempt at a description and comparison of VoIP and Digital.
Let us start with the definitions:
Descendants of analog telephones, digital telephones convert the analog voice signal into a digital signal and transport the signal across dedicated copper lines to a central switch. The digital signal utilizes a unique communications protocol, such as DCP to control the transmitting of the digitized voice signal as well as programming codes that reference additional features.
Digital telephones are limited (much like analog telephones), in that they must have a direct connection to the central switch. The direct connection can be through “home runs” or cross connect fields, but must be dedicated to the specific telephone.
An emerging (ok, it has been emerging for a long time now) technology in telephony is VoIP. This technology abandons the direct connection constraint of digital and analog telephone systems by leveraging the data network infrastructure. By changing the telephone into a network device, administrators can place telephones where ever the data network is available, without need for a direct connection to the central switch, so long as the centralized switch also has a connection to the data network. In the most simplistic sense, voice communications are converted by the telephone into digital signals which are then encapsulated for an Ethernet network and transmitted along side other Ethernet traffic.
While VoIP overcomes the limitations of digital telephones in the form of a direct connection, the new constraint is the quality of the data network.
Looking at it from the K.I.S.S. (keep it simple silly) standpoint, digital is a winner. The communications between digital telephones and the central switch rely on the standard copper wire that has been around for decades in one form or another. Trouble shooting is usually as simple as ensuring connectivity between endpoints and physical operation of the device. Although VoIP has come a long way, it is still vulnerable to many of the hazards associated with data networks, such as broadcast storms, high latency, and re-transmits. As more and more devices are added to a data network, the probability of the above mentioned hazards is enhanced.
Where there is a data port, there is a phone port. That simplicity makes VoIP the winner in the flexibility category. With additional expansion into Wi-Fi, there are few places that a VoIP phone can not be placed. Unfortunately, digital telephones really don’t even show up for this category.
This is probably the most difficult category to judge. On the side of VoIP, you have an almost limitless number of stations… so long as the data network can support it. For Digital, you can expand only as far as the physical facilities will allow. In the end, it comes down to the specific environment. Some facilities may have a decent data network and ample physical room. For them Digital is easier to expand. Some facilities have a highly tuned and scalable data network. For those facilities, the answer is VoIP. Overall, I think Expandability ends in a tie.
So, there you have it, my definitions and my assessment of Digital vs. VoIP.
I hope that helps!