ad hoc study
Mainly because of the general trend towards IP, SIP trunking will prevail in the long term and eventually replace ISDN. Furthermore, SIP trunking offers some advantages: granularity, centralisation and thus cost savings, although only large enterprises with many distributed offices can really benefit from these (i.e. aggregating the number of channels onto a few sites, instead of ISDN at all sites).
Carriers’ plans to switch off ISDN infrastructure will also become an increasingly important driver. However, there are also some obstacles. For one thing IP migrations (which typically include LAN, WAN, UC / PBX teams, possibly across several countries) usually take a (very) long time for the companies most likely to benefit from it, i.e. large enterprises with many offices. Besides, the still large number of installed TDM/ISDN PBX as well as the compatibility and security obstacles slow these migration processes further down.
We therefore overall expect, despite the recent ISDN retirement announcements, only a gradual constant growth over several years for SIP trunking, without any swift demise of ISDN or analogue connections, that may well stay in business for another five years or longer.