Its possible that there are too many users trying to use the same cell at one time.
Assuming its not the phone (eg, another phone is exhibiting the same problems),
have you called Shanghai Telecom and told them the issues?
They're usually interested in fixing issues like that.
I'd try another phone first though.
The bars on the phones are generally meaningless, at least one engineer at a phone manufacturer has admitted to faking it because if they put real signal strengths on there, the (american) providers got unhappy.
I can't be bothered looking that up again, but it came across as being quite believable.