Does anybody know if there is a cooling off period when signing contracts in HK? If there is what is the time limit?

We upgraded our 2G mobile accounts to 3G. The salesman assured us that the service would be just the same as before on our 2G phones but with more minutes and services etc. However, we have had nothing but problems, non connection, very long search times to find a network, getting cut off mid call, etc. and the staff at 3 have been useless so now we want to cancel.