There are regular mentions of this on the forums. Unfortunately the topics usually disappear off to private message with the forum team and an outcome or explanation is rarely shared in public.
If your phone was actually in use by you, would you have to plug into the back of the VM hub or a via conventional telephone wall socket?
Do you have any other equipment connected which might need to make use of the landline (some past topics have tracked the issue to things like call blockers, personal/medical alarms etc. dialling out unexpectedly and generating charges).