Identifying a restricted number requires using a service like TrapCall, which only works on mobile phones. Without such a service, these numbers come up as unknown and telephone providers cannot identify them. This can be a problem if the caller is harassing or threatening the receiver.
TrapCall is available with various plans and pricing options, and it doesn't matter what platform the cell phone is. Users can manage their accounts online their phones or computers, and there's an option to blacklist callers who are unwanted. As of 2015, the service is supported on the following carriers: Sprint, AT&T, T-Mobile, metroPCS, Verizon and U.S. Cellular.