Edit company greeting for option to call extension or voicemail

if you mean that you want there to be an input from the caller for example "press 1 for extension, press 2 to leave a voicemail" you need to set up a Multi-Level IVR.
So there are multiple steps to this, and multiple approaches. I am going to give you 1 example.
1. Create a voicemail extension (Phone System > Groups > Message only extension)
2. Create an IVR Menu (Phone System> Auto Receptionist > IVR Menus)
2.1 click on the created IVR Menu > Key presses > connect 1 to user ext. 301
connect key press 2 to the message only extension.
3. Switch from Single-Level to Multi-Level IVR (Phone System > Auto Receptionist > General Settings > upper Right corner)
4. when you chose multi Level IVR a pop up will appear asking you to chose an IVR Menu. Select the IVR Menu you just created.

To record the greeting for the IVR MEnu, you need to upload it first to the Prompt library(Phone System > Auto Receptionist > Prompt Library > Upper right corner
Once it's here you can use it in Phone System > Auto Receptionist > IVR Menus > Click on IVR Menu > Prompt > Audio