H.323 is really the beginning point for voip. (at least for standards based implementations) I would ping some of the folks that sit on the ITU group working on H.323. The timeline kind of works like this
H.323
IPDC
SGCP
MGCP
SIP
H.248
Of course, H.323 is up to V4 right now and IPDC and SGCP merged to MGCP 0.1 which became 1.0. Then SIP came somewhere between 0.1 and 1.0 and of course H.248 was ratified fairly recently.
Check the IETF and you can find drafts and their dates to get timelines on MGCP and SIP. H.323 if you can find anyone with access each document has a date assigned to it also.