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Since there is no single wireless network that can provide high system capacity and cost effective global service coverage for more demanding mobile users, existent heterogeneous wireless networks have to be efficiently integrated for this purpose profiting to an increasing number of multi-mode mobile nodes. Standard guard bandwidth allocation schemes, extensively studied and deployed in homogeneous cellular networks, don't take into account neither mobile node's multi-mode capability nor heterogeneous cells characteristics in future wireless networks. Thus, in our work, we consider a heterogeneous overlay wireless system implementing an extended guard bandwidth with an overflow scheme from various cellular layers and we model various CRTs (cell residence time) using general Gamma distribution that we specialize for each kind of cell taking into account its heterogeneous properties. Furthermore, we designed a fast online heuristic that regularly estimates the optimal guard bandwidth. Presented results, validated by computer simulations, shows that our proposed solution leads to a better wireless bandwidth utilization and a lower blocking rate while maintaining constraints on handoff dropping rates. The quality of these results are the consequence of the model accuracy used to estimate call-level QoS parameters and therefore the optimal guard bandwidth.