Protocol spoofing addresses this problem by having the local modem recognize that a data transfer is underway, often by looking for packet headers.
Before the introduction of echo cancellation in V.32 and later protocols, high-speed modems typically had a very slow "backchannel" for sending things like these ACKs back to the sender.
On the ~18,500 bit/s TrailBlazer, for instance, the modem could send as many as 35 UUCP packets a second to the receiver, but the backchannel offered only 75 bit/s, not nearly enough for the 35 bytes, 280 bits, of ACK messages generated by the remote host.
At the same time, the modem on the remote receiving end dropped the ACK packets being generated by the local computer's software, keeping the backchannel clear.
Since the channel efficiency only became a major problem at speeds over 2400 bit/s, and modems able to run faster than that typically had significant processing power anyway, protocol spoofing was mostly associated with these higher-speed systems.
A spoofing router or modem will intercept the SAP and RIP broadcasts, and re-broadcast the advertisements from its own routing/service table that it only updates when the link is active for other reasons.