You have a small problem: you've completely misunderstood everything!
https://en.bitcoin.it/wiki/API_reference_%28JSON-RPC%29
It says right there that it supports JSON-RPC over HTTP. Nothing at all about JSON-RPC over plain TCP (what you're calling "Telnet" protocol).
You say "It seems like you should be able to use telnet to manually make json-rpc calls" but you don't mention how you came to this conclusion.
JSON-RPC is typically implemeted over HTTP, but can be implemented over plain TCP, websockets, or any bi-directional internet protocol. That being said, the overwhelming majority of JSON-RPC server implementations expose their APIs via JSON-RPC over HTTP.
As others have mentioned, you can talk HTTP from a telnet
client and the payload of that conversation can be JSON-RPC.... if you know how. Just because you can doesn't meant you should.