CM18445 supports 2.5mm headset only. IPhone EarPods is with 3.5 mm Headphone Plug.

LeAnne, sorry to hear that you have difficulty in register the VH621 to your DS6251-2 phone system. Please check if the register steps below have been fully followed. If still unable to register your headset, our CS agents at 1-800-595-9511 can help. Our CS opens from 9am to 5pm, Mon to Fri.
Here are the headset register steps:
1. Make sure the battery is properly installed in the
headset. The LED light on the headset flashes
slowly in red and blue to indicate the headset is
not registered.
2. Place the headset in the charger cradle and
allow it to charge for at least one hour before
beginning registration.
3. Remove the headset from the charger cradle.
4. On the telephone base of the cordless phone,
press and hold /HANDSET LOCATOR,
FIND HANDSET or FIND HS, until the
IN USE light turns on and the screen displays the
“Registering” message, and then release the button.
The LED light flashes quickly in red and blue.
• If your telephone base do not have the
/HANDSET LOCATOR, FIND HANDSET or
FIND HS button:
Press and hold the HOLD button until the screen displays
Registering... Please wait, then release the button.
It takes about 60 seconds to complete the registration process. Once
the headset is registered, The LED light turns red when the battery
is low and being charged, or turns purple when the battery is well
charged, or turns blue when the battery is fully charged.
If the registration fails, you hear the voice prompt "Not registered"
when you press the ON/OFF button on the headset, and the ON/
OFF button light on the headset alternates slowly in red and blue. Try
the registration process again.

The system can support 6-party conference with a maximum of four outside calls
and two system extensions (including the initiating party).
There are two ways to establish a conference call.
Hold an intercom call
After establishing an intercom call with another system extension, the initiating
party holds the intercom call and invites an external party to join the conference
by making an outside call. Only one pair of intercom call can be held.
1. Press INTERCOM. Press or to scroll to the desired extension, then
MENU/SELECT or /SKIP. The called extension picks up the intercom call.
-ORPress the speed dial key assigned with the desired extension number. The
called extension picks up the intercom call.
2. Press an available line key to make an outside call. The original intercom
call is put on hold. When the external party picks up, you can have a private
conversation with that party.
3. Press CONFERENCE at any time to begin the conference call.
4. If necessary, repeat step 2 and step 3 above to invite another external party
to join the conference until the maximum capacity is reached.
Hold an outside call or join a call in progress
After establishing the first outside call, the party can invite another system
extension to join the conference by using intercom.
Any system extension can also join a call in progress by pressing the line key
which is currently in use to set up a conference.
1. Make or answer an outside call on the console.
2. Press another available line key to make another outside call. The original
call is put on hold. When the external party picks up, you can have a private
conversation with that party.
-ORPress INTERCOM. Press or to scroll to the desired extension, then
MENU/SELECT or /SKIP. The called extension picks up the intercom call.
-ORAny system extension can press the line key currently in use to create a
conference with the console and the external party.
If the call privacy of your console is set to On, follow the instructions of
To turn call privacy off during a call on page 29 to turn off the setting
temporarily.
3. Press CONFERENCE at any time to begin the conference call.
4. If necessary, repeat step 2 (make another outside call) and step 3 above
to invite another external party to join the conference until the maximum
capacity is reached.

Yes, CM18445 is a 4-line landline phone.