This letter proposes an analytic model to study the mis-routing problem cau
sed by user mobility in a wireless VoIP system. We derive the probability t
hat there are n mis-routed calls when a subscriber moves from IP network to
the GSM network. Our study indicates that if the user residence times in G
SM location areas, inter-call origination times and the inter-call delivery
times are of the same order, then the mis-routing effect can not be ignore
d.