I had the same issue when my iPhone updated to 9.1.311, Mitel released 9.1.312 the next day and I haven't had the issue since.
My server version is 9.1.2.102-01
There is an article about this in the Mitel KMS KB. AL273 - New iOS 13 SDK support in MiCollab iOS Client 9.1.3
Although it was only supposed to show a notification, not audibly ring. My experience was the same, phone would ring for a second or 2. It was annoying as hell.
Ask you vendor to search the above KMS article. I'd say you're going to need a patch and/or upgrade to you server.