Abstract: A mobile radiotelephone is responsive to user entry of an extension such as the last four digits of a telephone number, to identify a prefix such as the first three digits of a telephone number, for the user-entered extension. By automatically identifying a prefix, the user need not enter all of the numbers for the destination telephone. A default automatic prefix may be stored and retrieved in response to user entry of an extension. Alternatively, in response to user entry of an extension, the stored extensions in a memory such as a speed dialing memory are searched to identify a match to the user-entered extension. The associated prefix for the matched extension is retrieved. Default and speed dialing prefix identification may be combined. Simplified dialing and enhanced user safety are thereby provided.