Sony PCS-1 Operating Instructions Manual page 252

Message
Connection using SIP is not available.
Encrypted conference via LAN not
available.
Connection using SIP is not available. Line
is busy.
Transferring via a line interface other than
SIP is not available.
The operation is not available as some line is
in the hold mode.
252
On Screen Messages
Meaning
Connection using SIP cannot be done if "Signal
Encryption Methods" is set to "Standard
encryption".
Connection using SIP cannot be made as the
remote party has been connected to another.
You cannot transfer a call to the remote parties
connected via an ISDN or IP.
You can resume the call or disconnect the line only
in the hold mode.