It sounds like your mobile voicemail is kicking in before the landline voicemail and answering the call therefore our system sees it as an answered call and doesn’t send a missed call alert email.
To prevent this happening tick the “Require keypad press to accept the call” option in your Routing and save. You will then need to press any keypad key to accept a call. Since your mobile voicemail can’t press a key, it will no longer be able to answer the calls.
You can view your Routing page here: https://airlandline.co.uk/login