Typically for this behavior you would use a standard ring group. Here is a link to our guide on configuring ring groups:
The ring group needs to include both extensions 221 and 223 as members and be set to a type of "All Ring". Then you can use normal coverage to make both phones ring by coveraging immediately to the new ring group, and then to their voicemail. The screenshot below shows an example of this:
In this example someone who calls 2040 (this user's extension) will get routed to the ring group at 8001 that will ring both 2040 and another extension at the same time. After 4 rings with no answer the caller will then be sent to 2040's voicemail.