Well, think of it this way.

With all ETSI-GSM family networks, voice call is always "digital", however there are two ways to set up the voice call.

• 1.) The "traditional" digital telephony way, which is pretty much adaptarion from digital switching of the old-style non-mobile land telephony, usually called CS (Circuit Switched) call. This relies on SDH/ATM backbone and establishes a virtual switched circuit between the UE's. (This means there is a dedicated channel and timeslot for the call which is reserved for the duration of the call)
• 2.) The newer Packet Switched (PS) call which is just digitized voice call carried over IP/UDP packet stream, no preallocated routes needed.

The 2G and 3G networks have SDH/ATM backhaul and can establish CS calls, 4G and 5G cannot do this.

The 3G, 4G and 5G networks have IP backhaul and can establish PS calls. (and actually at least theoretically also 2G could do that because it could use VoIP over data connection altough I don't think that has ever been released in a public network)

The reason 3G is such a beast is because it has dual backhaul and also the additional burden of managing the power control over multiple cells which is required by the CDMA modulation scheme... compared to that 4G signalling is a walk in the park!

5G again gets a whole load of new stuff which ramps up the complexity (ultra-short feedback loops over air interface, adaptive antenna-array control, management of several modulation techniques simultaneously, etc...)