ERC1820 defines a universal registry smart contract that is required by ERC777.
ERC644 provides the upgradeability for CALL's smart contract; it is completely independent of the other two standards.
Is it necessary to use ERC644 and ERC1820 standards of tokens to work with ERC777 tokens? Or these standards of tokens are fully independent?
It is necessary to use ERC1820 as ERC777 requires it --->
https://eips.ethereum.org/EIPS/eip-777.
ERC644 is fully independent from ERC777. Our blockchain dev was able to integrate it bringing the capability of upgradeability to CALL's smart contract which in this new world of smart contracts and tokens, makes ours a true game-changer. Read more here --->
https://gcalliance.io/call-token/.