If you have 10 channels then if they are full will get busy.
You can have line keys, just cant separate them on one phone.
Calls will come in on first idle channel.
Look at max busy extensions, ICD Groups Misc Tab.
Group has 10 members, 5 ext get busy, next call that DID gets busy if I remember correctly.
Outbound CID is Wired ext,Ext Settings, ISDN CLIP Tab.

What about fax?