It seems that a recent update to the HikConnect app attempts to force the alert sounds out even if the device is set to silence notifications. As part of the way this has been implemented, it attempts to force the telephone into speakerphone mode, which can result in odd behaviour if you are mid-call when it happens, as it will effectively switch the call audio out through the speaker without warning!
I've reported it as a bug (but had no word back on a fix), but in the meantime you can workaround it by removing the "telephone" permission from the HikConnect app in the Android app settings permission controls. This should prevent it from being able to manipulate the telephone audio and, since I made this change, I've had no problems.
Shout if you want any more info on this problem
I've reported it as a bug (but had no word back on a fix), but in the meantime you can workaround it by removing the "telephone" permission from the HikConnect app in the Android app settings permission controls. This should prevent it from being able to manipulate the telephone audio and, since I made this change, I've had no problems.
Shout if you want any more info on this problem