I have the same complaints. Mitel told me to change the Call forward timer but that doesn't do it. No matter what value I put in there, the calls alway recall back to our internal voicemail after 15 seconds. But it is definitely a call rerouting problem because the only way to fix this is to turn off the call rerouting (however, the client can no longer use their internal voicemail). Even if changing the call forward fixed it, that is not a solution as it would make the internal voicemail unusable.