Is there any regulation preventing the use/sale of SIP trunks in mainland China?

I've set up and used commercial-grade SIP trunks in places where previously we would have used ISDN T1/E1 connections. Here in Shanghai I'm looking for a similar service, and while E1 30B+D services are readily available, every telecoms company we speak with says that SIP trunking is not available in China with re-sellers of both China Telecom and China Unicom. But no one seems to know why.

It seems logical to me that SIP trunks are cheaper to operate than ISDN services given that the first mile transit can be run over already-existing Internet infrastructure, and SIP signaling reduces the amount of configuration required by subscribers which is why it appeals to me. As such I've come to expect SIP services to be available in modern markets, and I've used them in quite a few countries.

For example, one place I know it's not possible is in India. Government regulations in India make it illegal to provide PSTN service using VoIP. (Citations: 1, 2). However it seems this may be changing. Perhaps China has something similar.