Not every implementation of uucico speaks and understand each protocol, so during the initial handshake phase, both processes have to agree on a common protocol. The master uucico offers the slave a list of supported protocols by sending Pprotlist, from which the slave may pick one.
Based on the type of port used (modem, TCP, or direct), uucico will compose a default list of protocols. For modem and direct connections, this list usually comprises i, a, g, G, and j. For TCP connections, the list is t, e, i, a, g, G, j, and f. You can override this default list with the protocols command, which may be specified in a system entry as well as a port entry. For instance, you might edit the port file entry for your modem port like this:
This will require any incoming or outgoing connection through this port to use i, g, or G. If the remote system does not support any of these, the conversation will fail.