It is ITU recommendation for Telecom operators how to form their numbering plans. Typically seen in cell phone networks when you dial your number starting with "+" sign which identifies E.164 number and allows to route calls properly to any country in the world. From regular phones (MITEL) it means using correct international prefix for country you originate call from. In simple words if you are in US, every call you start from 011, then country code, then city code and then desired number. If you are in Germany you start from "00" prefix. Since you are in control of your numbering plans in MITEL systems, you can make it E.164 compatible.
Normally for business applications all phones are assigned short internal numbers, which could be mapped to public (PSTN) numbers. For outgoing calls in business environment users must dial a specific prefix in order to distinguish internal numbers and public numbers. Common prefix in US is "9". In Europe it is "0" and very often combined with long distance prefix when calling outside of your local calling area.
