Merge some stability fixes from devel branch, REF 15852

Here is the list of commits included in the merge:
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commit fe6bedf1
Author: Yalu Zhang <yalu.zhang@iopsys.eu>
Date:   Tue Dec 10 15:10:16 2024 +0000
    Fix a bug that call clearing tones are not started when two other call parties hangup

    Call scenario
    - DUT calls A and the call is answered
    - B calls DUT
    - Press R to accept B
    - A hangs up and then B hangs up There are no call clearing tones after a brief beep.

    Cause
    handle_dialtone_timeout() calls chan_voicemngr_get_active_subchannel() to get the sub-channel
    and the sub[0] is returned and its state is ONHOOK. So the call clearing procedure is not
    started by handle_dialtone_timeout().

    Solution
    Let chan_voicemngr_get_active_subchannel() select the sub-channel with state CALLENDED over
    ONHOOK.

    Also remove the unused call state ANSWER.
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commit a6528cff
Author: Yalu Zhang <yalu.zhang@iopsys.eu>
Date:   Tue Nov 26 09:47:12 2024 +0000

    Set sub-channel state correctly when event RELEASE is received from voicemngr

    Set both sub-channels' state to ONHOOK if the current state is AWAITHOOK or CALLENDED.
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commit 58bc3e28
Author: Yalu Zhang <yalu.zhang@iopsys.eu>
Date:   Mon Nov 4 10:21:55 2024 +0000

    Fix indentation issues and remove some useless logs
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commit 44be72ac
Author: Yalu Zhang <yalu.zhang@iopsys.eu>
Date:   Fri Oct 25 15:11:58 2024 +0000

    Set connection as -1 in chan_voicemngr_close_connection() if there is other call on the line

    This is used to release all calls on a DECT extension. Otherwise there is still
    a remaining call on DECT when two participants have left the 3-way conference
    which is initiated by DECT.