As i have been lead to believe by trading standards officers, theres 2 parts of contratc, 1 part with network provider, for the service, then part with the agent, for things like cash back offers etc. so if the agent goes bust, the network provider isnt responsable for the cash back side.
But in this case, if the agent has said to the network provider the customer agreed to a new contract, when he hasnt. there in breach, so id would normally advise to get back in touch with then, in writing requesting a transcript of call. By all means contact the network provider, but they
prob just going to say nothing they can do, unless the agent contacts them.