Now I would feel that this might have been an obvious thing to check, especially if the call were being instead diverted to a number I had chosen. However, what I had chosen to divert all calls to was, in fact, no divert. It would seem reasonable that choosing the no divert option for all calls would try and send all calls to my phone. Instead, it meant that all calls went to an IVR sytem. Disabling voicemail, and disabling all redirects via GSM codes did nothing to address this. Likewise when I changed from "no divert" for "all calls" to simply "no divert" on each of the conditional forwardings, rather than receiving a call-failed situation or endless ringing the call was again diverted to the IVR which meant that in a situation where multiple phones were ringing, the IVR would always answer preventing me from ever getting a call. The bandage solution, as I've failed so far to explain the problem to the customer support, is to set conditional forwards to a number which will wait for a period of time, basically blocking the IVR. This is obviously sub-optimal.
Il vaut mieux hasarder de sauver un coupable que de condamner un innocent.