According to Salesforce Support, there is no way to overwrite the existing number on subscribers in mobile connect. By uploading an existing subscriberkey with priority 1 and a new number, this will get added as the primary number - but the old one will still exist as part of me.
In the short run, this is not an issue. But in the long run, given that numbers are recycled, we need a maintenance job to clear out priority 2 numbers.
Does any of you have experience of this via the API ? (indicated by the support team). I cannot find a documented way, when investing the mobile connect apis https://developer.salesforce.com/docs/atlas.en-us.mc-apis.meta/mc-apis/using-the-mobileconnect-api.htm