Hi, there’s already a gap listed in the tracker for retrieving tag–contact associations.
Once that work is implemented, you should be able to leverage the updated endpoint to retrieve the association data and then pull the additional details (like phone numbers) via the Contacts endpoint, or enrich from the Tags side as needed.
Also worth noting that REST v2 will not always map 1:1 with XML-RPC structures, so some differences in filtering (like multiple Tag IDs in a single request) are expected and may be handled differently going forward.