\mathrm{L}}  = \sum_{j=1}^{3} U_{l j} \, \nu_{j \mathrm{L}}~.
\label{3numix}$$ Here $\nu_{lL}$, $l  = e,\mu,\tau$, are the three left-handed flavor neutrino fields, $\nu_{j \mathrm{L}}$ is the left-handed field of the neutrino $\nu_j$ having a mass $m_j$ and $U$ is the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) neutrino mixing matrix [@BPont57; @MNS62]. If the neutrinos with definite mass $\nu_j$ are Majorana particles, the process of neutrinoless double-beta (-) decay, $(A,Z) \rightarrow (A,Z+2) + e^{-} + e^{-}$, $(A,Z)$ and $(A,Z+2)$ being initial and final state nuclei, will be allowed (for reviews see, e.g., [@BiPet87; @ElliotVogel02]). For Majorana neutrinos $\nu_j$ with masses not exceeding few MeV, the dependence of the -decay amplitude on the neutrino mass and mixing parameters is confined to one factor — the effective Majorana mass $\!\!$, which can be written in the form (see, e.g., [@BiPet87]): $${\mbox{$\left|  < \! m  \! > \right| \ $}}\!\! = \left| m_1 |U_{\mathrm{e} 1}|^2 
+ m_2 |U_{\mathrm{e} 2}|^2~e^{i\alpha_{21}}
 + m_3 |U_{\mathrm{e} 3}|^2~e^{i\alpha_{31}} \right|
\label{effmass2}$$ where $\alpha_{21}$ and $\alpha_{31}$ are the two Majorana CP-violating phases [^2] [@BHP80; @Doi81]. If CP-invariance holds, one has [@LW81; @BNP84] $\alpha_{21} = k\pi$, $\alpha_{31} = 
k'\pi$, where $k,k'=0,1,2,...$. In this case $$\eta_{21} \equiv e^{i\alpha_{21}} = \pm 1,~~~
\eta_{31} \equiv e^{i\alpha_{31}} = \pm 1 ,
