Technically this is functioning by design. When you decline a call it's going to go to your personal voicemail instead of the Smartline voicemail. Most similar services like Smartline that use call forwarding can only have the Voicemail accessed if you don't decline the call.
To avoid the call going to personal voicemail, either let the call ring through all the way or press the ignore button on the phone which will silence the ring but not decline the call. Both options will send the call to SmartLine voicemail.
Let us know if you have any additional questions.