- it is possible.
- you don't need "Shared lines"
- if you want to use the same phone number, you only need 1 license.
- the 2 handsets will be able to place simultaneous outgoing calls
- the 2 handsets will be able to be called individually.
- you won't be able to transfer a call from 1 handset to the other handset -> this sort of transfer would be possible with 2 licenses.
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The way it works is you are setting up the base station and not the handsets per se. If you are ordering from RC and the phone is assigned to an extension, then it should be plug and play. You would only need to pair the handsets to the base station.
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If it's a 3rd party device ordered from elsewhere you need to do the following:
Admin Portal > Phone System > Phones & Devices > User Phones > click on the user you want to assign it to. You will see a button "Change Phone".
Click on Change Phone > Bring your own device > enter the model of the phone > follow the instructions for provisioning the device.

Pair 1 handset to the base. Then pair the 2nd handset.
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You mentioned it is a large space. Not sure how big is "large" for you, but you might want to consider range. Most likely you'd want to place the base station in the middle, so it has best coverage. Once you place it, it would be good to do a test by placing a call and walking around in the area, while checking if the quality is good/ if there are any cracks. Try to see if there are any "dead spots" where the signal might get distorted. If there are quality issues, make sure the base is placed on an elevated surface and not hidden behind furniture/ inside cupboards.