Resistivity $\ensuremath{\rho}$ and thermoelectric power $S$ as a function of temperature on samples of nominal composition (Bi${}_{0.8}$Pb${}_{0.2})$${}_{2}$Sr${}_{2}$Ca${}_{3}$(Cu${}_{1\ensuremath{-}x}$Si${}_{x})$${}_{4}$O${}_{12\ensuremath{-}\ensuremath{\delta}},$ for different $x$ values at small concentrations, have been carried out. We find a correlation between $\ensuremath{\rho}$ and an enhancement in $S$ observed above ${T}_{c}.$ We analyze the results in terms of conventional theoretical models that include vertex and other electron-phonon $(e\ensuremath{-}p)$ corrections to the scattering due to impurities. It is argued that in the copper oxides the $e\ensuremath{-}p$ effects, usually negligible in normal metals at temperatures small compared to the Debye temperature, might be significant near and above ${T}_{c}.$ A small contribution due to the phonon drag effect is also discussed. It is suggested that the trends of our results adjust to the predictions of those conventional models.