This should be fairly easy to troubleshoot and fix.

Start at the beginning and trace the call routing. Dial the CRA extension numbers directly and listen to the greetings. Look for different greetings assigned to the CRA in day and night mode. Check the digit translations, and make sure there is an Extension ID or mailbox assigned to the device you’re trying to dial.

As to double greetings; the same greeting is recorded twice. Just figure out where. It’s either recorded on the CRA that times out to the mailbox, or someone recorded the greeting as their name in the mailbox (so you hear it twice).

Watch the display on your phone. When you test this via an intercom call, the display will tell you if you’re listening to a CRA or mailbox greeting.

For dialing zero; when are they dialing zero - from a CRA or mailbox? Is ext 1700 manually forwarded to voice mail? Are there STAR applications?

Figure out what your issues are, and tackle them individually. The answers should jump right out at you.