As we can see in the above diagram, aprt from transport technology migration from TDM to "All-IP" LTE/4G architecture retains many similarity to 1G - AMPS Network.
The major difference is that the Central Core (i.e. MTSO) in AMPS was handling both User and Control Planes. But in 4G they are separated to allow independent growth and modernization along with extensive reduction in "LATENCY".
LATENCY was not at all a factor during 1G or Initial 2G Periods.
Since LTE is more of a "DATA" network and supports Voice in the form of VoIP, it provides the flexibility in using the Voice CODECS depending upon various factors. But 2G Networks are still having a fixed rate/codec (sets) due to their radio access technology.
Eventhough GSM networks are suffering the lack of flexibility like LTE/LTE-A/3G networks, there are some steps taken by major vendors to revive this market based on the interest from existing 2G Only operators/ the ones who has a license in hand.
There are many hurdles in the form of existing 2G operator to become a LTE Operator. May be a regulatory/license obligation/business viability can halt them.
From my opinion, Voice is still going to rule for a while in the form of 2G Networks.The proof, Single Radio Voice Call Continuity (SRVCC) /VoLTE are developed not only to provide a smooth migration, also will help 4G Operators to offload all their Voice Service demand from their customers to 2G Networks.
So Technology oriented licensing which cornered the 2G Operators hsa a greater chance in reviving their business in the form tie ups with 4G Operators.
Reference :Telefónica makes first multivendor VoLTE-to-3G call handover
Reference : US Wireless Carriers Move to New Technology—for Voice
