In response to the decline of landline households and the growth of cellphone-only users, more and more telephone surveys conduct interviews by cellphone as well as by landline. However, in the absence of a sampling frame that covers both landline households and cellphone users, a dual-frame sampling design is inevitable, that is, sampling from a landline frame and a cellphone frame separately, and then combining the two samples for the analysis. These two frames overlap in the subpopulation that uses both a landline and cellphone, and the sampling rates of them are usually different.