Add support for single/multiple call mode

Since BT wants the call mode (which can be set on DECT handset per line) should be applied for FXS as well I had to do changes in voicemngr and asterisk as well. This is asterisk part for Your review Yalu. release-6.5 branch has been updated as BT was pushing for solution, but for devel I want You first review and we could decide if we merge it or not. Corresponding voicemngr merge request: voicemngr!11 (merged)

dectmngr changes which are in devel already: https://dev.iopsys.eu/voice/dectmngr/-/commit/27f69239e95c7e7c2c9673825bdafde446ae5ae2 https://dev.iopsys.eu/voice/dectmngr/-/commit/fbdd29485aec30b930d1560ff3c8f83e7f483dcb https://dev.iopsys.eu/voice/dectmngr/-/commit/2465b94d72b8d54102c10565e624847d4f0b39ee

for more details please see the ticket: https://project.iopsys.eu/issues/11142