Ok, what I would do so as not to inconvenience everyone else connected to the phone system is to:
1) Create an Omegatrek or use an available SLT port and set it up as a Phantom Extension, using any un-used extension number.
2) Either direct the DID to this phantom extension number in all Day and Night modes, or to a station group that includes the phantom extension as a member.
3) Add a call cover key to any phones that need to answer calls from that DID.
If you use the group method, then inbound calls will show Caller ID and answering the call only requires lifting the handset. If the DID is directed straight to the phantom extension, then the phones will not show CID and the key will need to be pressed in order to answer the call.
If this is confusing to you, then you really need to have a certified technician come in and make this change for you as there are various consideration to take into account along the way, including setting up the voice mailbox and setting up a Multiple Mailbox key.