Not all MultiLine users will get voicemails picked up by their personal voicemail. There is an optional feature that organizations can implement to prevent it.
When MultiLine uses your regular cellular service and the minutes in your carrier plan ("Minutes Mode") to receive inbound calls, if you decline the call or if your phone is switched off/has no signal, voicemails may show up in your native voicemail.
This is because, in this case, there isn't a difference technologically between you answering the call and your personal voicemail system answering it.
If you let the phone ring or silence the ring, the call will go to MultiLine voicemail. You can silence the ring by using the volume key.
When MultiLine uses mobile data or Wi-Fi ("Data Mode"), the voicemails will always show up in your MultiLine voicemail.
You can tell whether an inbound call was received using Data or Minutes by looking at the Call Icon.
When using Minutes Mode, certain native carriers may divert calls to native voicemail before MultiLine voicemail can answer.