@ AKroger
I followed those first steps originally, but after I dial the access code I entered into the Feature Access Code Assignment page, then dialled the extension to forward to, my phone then double-beeps, and I noticed if I wait a while it just displays 'invalid'
The phone is a Mitel 5207

In response to the other settings
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Ensure that the forwarding station and the destination station are permitted to connect to one another in the Interconnect Restriction Table.
-There are no entries in this list, if I click 'Change' I get a screen with all entries saying 'not restricted'

For Call Forward Follow Me (Third-Party) and I Am Here forwarding, enable Call Forwarding Accept in the Class of Service Options Assignment form of the forward destination.
-I don't see this anywhere?

Program the Number Of Forward Hops in the System Options Assignment form.
-The number of forward hops is set to 2

To allow forwarding to an external destination, enable Call Forwarding (External Destination) in the Class of Service Options Assignment form.
To allow forwarding to an internal destination, enable Call Forwarding (Internal Destination) in the Class of Service Options Assignment form
-I'm trying to forward to an internal extension, the option is set to yes

For Call Forward Follow Me, enable Group Call Forward Follow Me Accept and Group Call Forward Follow Me Allow in the Class of Service Options Assignment form.
For Call Forward Follow Me (Third Party), enable Third Party Call Forward Follow Me Allow in the Class of Service Options Assignment form for the remote telephone.
-I can't see this in my list


Call Forwarding Busy (Ext & Int Source)

Cancel Call Forwarding Busy (Ext & Int Source)

Call Forwarding Busy (External Source)

Call Forwarding Busy (Internal Source)

Call Forwarding No Answer (Ext & Int Source)

Cancel Call Forwarding No Answer (Ext & Int Source)

Call Forwarding No Answer (External Source)

Call Forwarding No Answer (Internal Source)

Call Forwarding Follow Me

Cancel Call Forwarding Follow Me

Cancel Call Forwarding Follow Me Remote

Call Forwarding Follow Me Third Party

Cancel Call Forwarding Follow Me Third Party

Call Forwarding I Am Here

-All of these are set
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@ JimmyV
I get:

IP Device ID: 71
Circuit Location : 1 3 1 71 1
Extension : 928
Active Features :
Call Forward No Answer External : 913
Call Forward Busy External : 913
MAC Address :
(MAC address listed)

When I call the number from an external number though, when I don't answer it goes to my voicemail rather than forwarding the call.

(Once again, really appreciating the help!)