Femto cells have been a buzzword for many years in the industry. They might now have been renamed into small cells but in most countries they are not (yet) used. One way to use a femto cell is to patch coverage holes in private homes. Here, it may makes sense to restrict the use of the femto to family members. Other mobiles that see the femto and try to use it are then rejected and have to return to the macro network. There are several ways how that can be done and I always wondered which one is used in practice. Recently, I stumbled over this blog entry that describes an encounter with a femto in practice. This one used a Location Update Reject with Cause Code #13 (Roaming Not Allowed In This Location Area). This then triggers the return to the macro network. For further details, have a look at that post, it also contains other interesting thoughts about femtos in practice based on the authors observation.