3G/WLAN integration is thought to be a viable solution for next generation heterogeneous access networks. Estimation of traffic in 3G and WLAN systems is essential to compute the performance of a 3G/WLAN mixed cell. The sharing of calls between 3G and WLAN is dependent on call intercepting (CI) probability of WLAN inside a 3G cell. CI probability is the key parameter for model based analysis of capacity gain and call dropping performance of a mixed cell. Most of the existing models do not derive CI probability. Rather the analysis has been done on the basis of assumed values of CI probability. In this article, we propose an analytical model to derive the CI probability as a function of some realistic cell parameters. The model provides linear changes of CI probability with increasing number of WLAN-hotspots. The model facilitates to represent all WLAN-hotspots by an equivalent single WLAN. Proposed model removes the error in calculating CI probability by 7.6% over existing models. The numerical results are useful to estimate the net traffic in 3G and WLAN systems of mixed cell. © 2012 Springer-Verlag.