| OCC - Other Common Carrier | Not part of the original AT&T system. |
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| Off Hook | The signal that the telephone receiver has been lifted (activated). Originating off hook activates a dial tone on switched networks. Destination off hook completes a call (and activates minute-by-minute billing for long distance calls). |
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| One Plus - 1+ | Customer ability to access the long distance service provider of their choice by first dialing 1, then the long distance number. Equal Access guaranteed by the 1982 AT&T MFJ. 1+ is an outbound service where the calling station pays the charges. |
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| Operator Service Call - OSC | A call that is placed through a human or automated operator (0+). |
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| Operator Service Provider - OSP - OS Provider | The vendor that supplies operator service. |
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| Outbound | Outward Sending - Call Originating - Dialing Out |
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