In simple terms it all comes down to the SIMs. The operators keep an eye out for SIMs that are making an excessive amount of calls from one location. If this SIM is in use too much it is obvious that it is being used for wholesale in a GSM Gateway. The operator will then turn that SIM off and you’re stuck with a useless SIM.
GSM channel bank operators are always one step ahead of the game though. By only using the SIM for a couple of hours a day, basically having a high number of SIMs and rotating their use, it reduces the risk of being cut off. People also move their SIM’s between different locations so they appear to be in phones not gateways – obviously this only works if you have multiple locations etc.
Another trick is to send the odd text message to the SIMs – this again gives them the appearance of being in normal operation.
In the UK the operators have really cracked down recently and although they are not actively prosecuting people they are turning off large batches of SIM cards. This has had an effect on the market that the cell/mobile pricing has gone up by 2-3 pence per minute on the wholesale market.
I hope that this answers some of your questions – if you want any further information please feel free to contact me at: email@example.com