Not looking for an explanation as I prefer it to be magic.
Its is magic, somewhere in the Sim and or Phone settings.
At some point you have probably put your lobster Sim (the 717 VM one) into the Wifes Phone, Depending on the Network and Phone, a Voice Mail incoming Call flags up as a Diverted Call and may or may not have a ID Flag in the ID String as to the Originator, ie, +447836121121 or 121, 123, 717, or just "Answer" or "Voice Message" etc, again depending on the Phone, it will remember the setting from when the Sim was inserted and store the info, also putting that number into the Short Dail No. 1. just to make it more difficult international Voice Mail calls and numbers get handled differently when Roaming, again dependent on the Network and whether you have Roamed onto a Partner Network or not.
So bottom line is your Wifes Phone when recieving a incoming Voice Mail Call from Three has 717 stored in the Phone somehow.
If its not that, I have no idea what it is so best just to tell the Wife to chuck it in the bin.