After switching network providers, your previous provider should send you a copy of your "final bill". The time frame on this can vary, it can take anywhere from 14 days to 6 weeks depending on the provider.
If you are an Ice mobile customer, you can claim your money back on this simply by sending over a copy of the bill. The accounts department will then allocate this to your invoices by way of a credit. By way of an example, if your final bill was £200 and your monthly payments to Ice comms were £100, you just wouldnt pay Ice comms for 2 months.
If you are billed directly by the network then the process is slightly different in that we not only require a copy of the final bill but a corroborating invoice from your company to go alongside it. This then gets sent to the finance department where this gets scheduled in for payment directly in to your bank account.