AT&T PARTNER Installation And Use Manual page 27

If Privacy is active at a combination extension, no other extension can join
a call in progress at either the system phone or the standard device
connected to it. (However, a phone can interrupt a call on the other
phone or standard device at that same extension. For example, if a
system phone and modem are combined at an extension, picking up the
handset of the system phone could interrupt a modem transmission.)
The PARTNER headset is a tip/ring device (equivalent to a standard
phone) and can be used in combination with a phone.
3-10 Learning About Telephones