Call Route / Forwarding

Hi, I am very new to this voice domain.. I have a requirement. One of my customer is having Call Manager 8.X in his network. He wants his GM to receive all the internal calls directly, where as the calls from PSTN should reach the Secretary first and the secretary should forward the calls to GM. The last four digit of the DID is used as the extension number and both GM and secretary are using single line each (not a shared line). GM’s Phone model is 9951 and secretary’s 7965. Please help on this requirement.

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