Polycom RMX 1500 Administrator's Manual page 76

Chapter 2-Additional Conferencing Information
2-14
The participants Video Protocol in the New Participant or Participant
Properties - Advanced tab should be left at (or set to) its default value: Auto.
The Auto setting allows the video protocol to be negotiated according to
the endpoint's capabilities:
• OCS endpoints and Lync Server clients connect to the conference
using the RTV protocol.
• Other endpoints negotiate the video protocol in the following
sequence: H.264, followed by RTV, followed by H.263 and finally
H.261.
Protocol Forcing
Selecting H.264, RTV, H.263 or H.261 as the Video Protocol results in
endpoints that do not support the selected Video Protocol connecting as
Secondary (audio only).
Auto