The zero prefix is the initial digit dialed in many national telephone plans to signal that the call is leaving the local exchange and requires trunk routing. When dialing internationally into these countries, callers must drop this prefix after the country code. It acts as an escape character for domestic long-distance routing.
The PBX is failing to route the call because it's stripping the zero prefix before hitting the national trunk.
A common error in international dialing is keeping the zero prefix after the country code, which causes the routing switch to reject the call.
Related terms
Prefix
The initial digits of a telephone number that route the call to a specific geographic region or service type.
Trunk call
A historic long-distance telephone call routed through major intercity exchanges.
Subscriber trunk dialling
The automated telephone exchange system that allows callers to dial long-distance numbers directly.
Escape character
A specific control code that changes the meaning of the data immediately following it.