How to equalize an ISI channel when the transmitted symbols are unknown?
The classic approach is to include a "training sequence" along with the data. The training sequence is known to both transmitter and receiver, so it can play the role of $\mathbf{x}$ in your question.

Now, in practice most wireless channels fluctuate over time. This means that the vector $\mathbf{h}$ is not fixed, but changes randomly in time. If the symbol rate is not too high, then the channel will remain essentially fixed during the transmission of several symbols. This time is called the "channel coherence time". When the symbol time is much smaller than the coherence time, we say the the channel exhibits "slow fading".

Over a slow fading channel, the training sequence is repeated approximately once every coherence time. During one coherence time interval, the equalizer remains fixed.