List of international call prefixes
International dialing prefixes are used when dialing international telephone calls by telephone users. These prefixes are typically required only when dialling from a landline, while in GSM-compliant mobile phone (cell phone) systems, the symbol + before the destination country code may be used irrespective of the location of the telephone when dialing.
Source: Wikipedia — List of international call prefixes (CC BY-SA 4.0)