The flowchart above describes the following "double" call-hold scenario:
1. A calls B and establishes a voice path.
2. A places B on hold; A hears a dial tone and B hears a held tone.
3. A calls C and establishes a voice path.
4. B places A on hold; B hears a dial tone.
5. B calls D and establishes a voice path.
6. A ends call with C; A hears a held tone.
7. B ends call with D.
8. B retrieves call with A.
Notes:
• If a party that is placed on hold (e.g., B in the above example) is called
by another party (e.g., D), then the on-hold party receives a call waiting
tone instead of the held tone.
• While in a Double Hold state, placing the phone on-hook disconnects
both calls (i.e. call transfer is not performed).
• You can enable the device to handle incoming re-INVITE messages with
"a=sendonly" in the SDP, in the same way as if "a=inactive" is received
in the SDP. This is configured using the SIPHoldBehavior parameter.
When enabled, the device plays a held tone to the Tel phone and
responds with a 200 OK containing "a=recvonly" in the SDP.
28.2 Call Pickup
The device supports the Call Pick-Up feature, whereby the FXS user can answer someone
else's telephone call by pressing a user-defined sequence of phone keys. When the user
dials the user-defined digits (e.g., #77), the incoming call from the other phone is forwarded
to the FXS user's phone. This feature is configured using the parameter KeyCallPickup.
Note: The Call Pick-Up feature is supported only for FXS endpoints pertaining to
the same Trunk Group ID.
28.3 BRI Suspend and Resume
The device supports call suspend and resume services initiated by ISDN BRI phones
connected to the device. During an ongoing call, the BRI phone user can suspend the call
by typically pressing the phone’s “P” button or a sequence of keys (depending on the
phone), and then on-hooking the handset. To resume the call, the phone user typically
presses the same keys or button again and then off-hooks the phone. During the
suspended state, the device plays a howler tone to the remote party. This service is also
supported when instead of pressing the call park button(s), the phone cable is
disconnected (suspending the call) and then reconnected again (resuming the call).
If the phone user does not resume the call within a user-defined interval (configured using
the HeldTimeout parameter), the device releases the call.
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