Yealink SIP-T48G Administrator's Manual page 264

Microsoft Lync Edition Administrator's Guide for SIP-T2_Series and T4_Series IP Phones
Capturing packets after you enable the RFC 2543 Hold feature. SDP media direction
attributes (such as a=sendonly) per RFC 2543 is used in the INVITE message when
placing a call on hold.
Capturing packets after you disable the RFC 2543 Hold feature. SDP media connection
address c=0.0.0.0 per RFC 3264 is used in the INVITE message when placing a call on
hold.
For more information on RFC 2543 hold feature, refer to
information on capturing packets, refer to
252
Call Hold
Capturing Packets on page 241.
on page 132. For more