  Asymmetric tri-bi-maximal mixing is a recently proposed, grand unified theory
(GUT) based, flavor mixing scheme. In it, the charged lepton mixing is fixed by
the GUT connection to down-type quarks and a $\mathcal{T}_{13}$ flavor
symmetry, while neutrino mixing is assumed to be tri-bi-maximal (TBM) with one
additional free phase. Here we show that this additional free phase can be
fixed by the residual flavor and CP symmetries of the effective neutrino mass
matrix. We discuss how those residual symmetries can be unified with
$\mathcal{T}_{13}$ and identify the smallest possible unified flavor
symmetries, namely $(\mathbb{Z}_{13}\times\mathbb{Z}_{13})\rtimes
\mathrm{D}_{12}$ and $(\mathbb{Z}_{13}\times\mathbb{Z}_{13})\rtimes
\mathrm{S}_4$. Sharp predictions are obtained for lepton mixing angles, CP
violating phases and neutrinoless double beta decay.
