When secretary transfer call to an iphone and the call goes to the voicemail

Hello

Please be kind my english is not to good

We have a problem whit Iphone voicemail.
When secretary transfer call to an iphone and the call goes to the voicemail most of the time the message does not recorded in the Bell Canada voicemail system.
Check whit Bell they say it’s are system.When i tried i could see that when i hang up the trunk that is used will take more then 15 seconds to hang up. Could it be the problem?

i have check whit the company who installed the system and they seem to not know what to do"

Please help me.

thanks to all responds.