Mobile networks in Europe, Asia (except Hong Kong, Macau (Macao) and Singapore), Australia, and Argentina only charge their subscribers for outgoing calls. Incoming calls are free to the mobile subscriber with the exception of receiving a call while the subscriber is roaming as described below. However, callers typically pay a higher rate when calling mobile phones. Special prefixes are used to designate mobile numbers so that callers are aware they are calling a mobile phone and therefore will be charged a higher rate.