[Answer]-Django: relevance of where I put the ForeignKey fields and whether or not I should use the admin site

I’ve no idea why you want to define things this way round. The way you want to have it, one phone number can belong to multiple clients, which seems unlikely. Having the ForeignKey field on the PhoneNumber model is the right thing to do, because that way the relationship makes sense (a client can have multiple phone numbers).

But there’s no need to muck about with abstract models and subclassing. If you need the ability for a phone number to point to multiple different model classes, then generic relations are what you need.