Any VoIP handset which supports SIP will normally work with the FB2900. Most makes of handset actually allow multiple identities on the handset so it can appear as multiple handsets to one or more phone systems, but a typical installation will not normally need more than on identity per handset.
On the handset you will need to set a registrar and/or proxy which is usually either a host name or an IP address. This will need to refer to the FB2900's address.
The handset will also have some form of login or username and a password. Typically you would use the extension number or DDI as the username, but in an office PABX you may want people's names as the user name.
On the FB2900 you add a telephone configuration object (VoIP users) for each telephone,
specifying a username and password. If the handset will need to
connect from somewhere that is not on the local network (LAN) then you also need to set local-only
to false.
name field matching the SIP request. However,
if this is not matched and the telephone has sent a pre-challenge Authorization header to indicate
the username it is using then this will be matched instead.
You also need to set the extn and ddi for the phone.
Also, in order to make external calls you need to select a carrier to use.
The VoIP status page shows the active registrations from handsets.
log-register and log-register-debug settings can provide a lot
of information about registrations and help diagnose any problems.