  In a recent work by Gonz\'alez-P\'erez, Parcet and Xia, the boundedness over
non-commutative $L_p$-spaces of an analogue of the Hilbert transform was
studied. This analogue is defined as a Fourier multiplier with symbol $m\colon
\mathrm{PSL}_2(\mathbb{C})\to \mathbb{R}$ arising from the action by isometries
of $\mathrm{PSL}_2(\mathbb{C})$ on the $3$-dimensional hyperbolic space
$\mathbb{H}^3$. More concretely, $m$ is a lifting of the function on
$\mathbb{H}^3$ that takes values $\pm 1$ in the two regions formed by dividing
the space by a hyperbolic plane. The boundedness of $T_m$ on $L_p(\mathcal{L}
\mathrm{PSL}_2(\mathbb{C}))$ for $p\neq 2$ was disproved by Parcet, de la Salle
and Tablate. Nevertheless, we will show that this Fourier multiplier is bounded
when restricted to the arithmetic lattices
$\mathrm{PSL}_2(\mathbb{Z}[\sqrt{-n}])$, solving a question left open by
Gonz\'alez-P\'erez, Parcet and Xia.
