  We investigate the phase coherence of isoscalar pairs from the $B({\rm
GT};0^+_1\,T\!=\!1 \to 1^+_1\,T\!=\!0)$ values in two-particle configurations
of $A=6$, 18, and 42 nuclei and two-hole configurations of $A=14$ and 38 ones.
We find that these Gamow-Teller (GT) matrix elements are always constructive
and thus enlarged under isovector- and isoscalar-pairing Hamiltonians, whereas
the observed GT strengths are strongly hindered for the two-hole
configurations, including the famous $^{14}$C dating $\beta$ decay. This
indicates that the actual isoscalar pair, unlike the isovector pair, has no
definite phase coherence, which can work against forming isoscalar-pair
condensates.
