5.16.5. Indirect Network Access 
Example: Indirect network access to private network
In general, the requirement applies of routing local calls via PTT
and long-distance calls via other Carriers. Access to the Carrier can
be realized directly or indirectly via PTT.
Indirect access:
Figure 29. Indirect Network Access
Call setup procedure:
Station dials the number: 08972239500. The switch seizes a trunk circuit
(digital) and sends the following digits:
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131
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CO call to private network. Dialing the D channel
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Dial tone
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System waits for second dial tone
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Authorization code
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Authorization code for network access, if required
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08972239500
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Number
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The authorization code and the nummer are sent via the DTMF signaling
in the B channel.