<ol>
<li>Pick an extension. I like to pick the lowest-numbered one.</li>
<li>Set up the call forwarding on that extension by adding a New Number and select Business. (I don't know if it matters which option you select)</li>
<li>Remove the main phone number association from whatever it's attached to</li>
<li>Go back to the extension and assign the main number to it.</li>
<li>Repeat 3-4 for any other phone numbers that need to be forwarded.</li>
</ol>

 1. Pick an extension. I like to pick the lowest-numbered one.
 2. Set up the call forwarding on that extension by adding a New Number and select Business. (I don't know if it matters which option you select)
 3. Remove the main phone number association from whatever it's attached to
 4. Go back to the extension and assign the main number to it.
 5. Repeat 3-4 for any other phone numbers that need to be forwarded.

Vonage Call Forwarding

8x8 Call Forwarding

Comcast Call Forwarding

3CX Call Forwarding

adit Call Forwarding

AT&T Call Forwarding

Century Link / Lumen Call Forwarding

Cox Call Forwarding

Cyrtacom Call Forwarding

Frontier Call Forwarding

How to Enable Call Forwarding (Onboarding Guide)

Giant Communications Call Forwarding

GoToConnect (Jive) Call Forwarding

Google Voice Call Forwarding

Grasshopper Call Forwarding

Intermedia Call Forwarding

Nextiva Call Forwarding

Peerlogic Call Forwarding

QuestBlue Call Forwarding

RCN Call Forwarding (Astound Broadband)

RevenueWell Call Forwarding

RingCentral Call Forwarding

Spectrum Call Forwarding

Thread Communication Call Forwarding

Verizon Call Forwarding

Weave Call Forwarding

Windstream Call Forwarding

Onboarding