Abstract: In a system for checking the access to data of a data unit (T10, T20, T30), the data unit is connected to a public network (V2) and is capable of exchanging data with a second subscriber (T4) over this network. In order that an authorized subscriber has access to the data of the data unit even without a secret telephone number, an adapter (T15, T25, T35) is allocated to the data unit (T10, T20, T30), which adapter can be dialled over a secured network (V1). The adapter (T15, T25, T35) is arranged in such a way that, in response to being dialled over the secured network (V1), the adapter causes the data unit (T10, T20, T30) to dial the second subscriber (T4) whose telephone number is stored in a memory (T11, T21, T31) allocated to data unit.