You don't want to use the (inbound) Marketo API. You want to look up your leads against the DNC list, not the other way around, and you'd never have enough API calls to do this inbound (and it would kill your instance even if you did).
You'd use a webhook (i.e. Marketo's outbound API) for this. The webhook platform could reach out to each of these databases (in parallel for speed) for each of your leads. A one-time backfill will take care of the current database, then you run the webhook again when a new lead comes or a phone field changes.
Thanks Sanford. That makes perfect sense.
I am also trying to identify all the B2B DNC list across all EU countries. So far, it has been challenging to find an aggregation of it so we can have one vendor for it all.