A wireless telecommunication system comprises a base station and a terminal device (UE). The UE is configured to determine its mobility status and an associated reliability, e.g. an indication of the extent to which the UE is moving and the confidence in this determination. In some cases the reliability may be based on how the UE determined its mobility status. For example, if the UE determined itself to be of relatively low mobility because it has an internal configuration setting indicating it is deployed in a fixed position, the reliability of this mobility status may be considered to be greater than if the UE determined itself to be of relatively low mobility because it has not recently undergone many base station handovers. The UE is configured to convey an indication of the mobility status and the reliability to the base station. The base station may then configure the UE for operation within the wireless telecommunications system in a manner that takes account of both the mobility status and the reliability for the mobility status. This can help optimise configuration settings for the UE which depend on device mobility.