Multimode terminals designed for next generation wireless networks (NGWNs) have the capability to support two or more classes of calls simultaneously. When a multimode terminal having an ongoing call initiates a new class of call, the current RAT may not support the new class of call thereby necessitating a vertical handoff to a RAT that can support both the current and the newly initiated call. Thus call dynamics of a multimode terminal affects RAT selection in heterogeneous wireless network. However, the effect of call dynamics of a multimode terminal on RAT selection has not been considered in the literature. Therefore, this paper focuses on the above subject. We develop an analytical model for call dynamics of a multiservice multimode terminal, and investigate the effect of call dynamics of RAT selection in heterogeneous wireless network, in terms of the probability of having necessary vertical handoff. Some results are presented to illustrate the effect of call dynamic on RAT selection.