First of all: A cancellation is different to a withdrawal -- main difference: A cancellation will become effective after the end of the minimum term whereas a withdrawal (if possible) will become effective immediately. So if you really cancelled the contract - even though you returned the devices - you are still stuck to the contract until the minimum term is over -- a possible refund from the returned device will be placed against your monthly invoices.
If it anyhow was a withdrawal, it might nevertheless take some time for the in order to get the refund processed -AND- due to German laws preventing money landering, the refund will usually only be granted using the method you originally paid for, e.g. if you used a bank transaction, it'll be paid back there.
And keep in mind that -even if it was a withdrawal- you still have to pay the partial monthly fee for the time between signing the contract and returning the device as well as extra charges from the usage (e.g. data top ups, roaming, SMS, ...).
