Hi Kristen,
I agree with @Paul Chase that setting up a Ring Group will be your best bet. Once the group is setup, you'll designate a specific phone number which will ring to it.
Then you'll determine whether all members of your Ring Group will ring simultaneously when calls are presented, one person at a time, etc.
What I like about this setup the ability to share a single voicemail account for the group.
That way you and your colleague don't have to worry about maintaining messages between individual accounts.
Please share if this helps with your issue. Thanks!
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Jahmal Abbott
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