`delivery_class` is optional in v02 jsbox conversations too. If `delivery_class` is not set in the config, we give `Contact.for_addr` a `None`, so it returns the contacts msisdn: https://github.com/praekelt/vumi-go/blob/develop/go/vumitools/contact/models.py#L77-L80

It does feel like the code here is relying on an obscure part of code living somewhere else, not really sure how to do this better though. I'm up for suggestions.