Call forwarding sends the incoming call to a configured phone number or to several numbers if you have multiple agents handling calls. Once an agent answers the call, all of the other ringing devices (of the other agents) will stop ringing.
Having this in mind, please check the following facts:
An agent with a turned off phone will always win the call because that phone will always answer the call as soon as it comes in to send it to the voicemail of that number. Disabling call forwarding when the phone is turned off will fix this issue.
A number hosted by a PBX or an automatic answering machine has the same effect as above because it answers the call as soon as it comes in.
Rejecting a forwarded call will answer the call with the personal voicemail message of that number. It has the same effect as answering the call and a way to prevent this from happening is to let the call timeout or get missed.
A voicemail answering a call is no different than a human answering a call. That's why it cannot be detected and prevented by Toky.