This paper addresses the challenges related to the interoperability of GSM and IP networks by presenting an interworking concept between the GSM and IP telephony services. The Mobile IP protocol provides mobile Internet hosts with data service roaming between IP sub-networks of different access technologies. Being a generic solution, Mobile IP may fail to exploit the special characteristics of some access technologies. In particular, Mobile IP is not a sufficient solution for the speech service roaming between GSM and IP telephony. The presented concept provides transparent mobility for both data and speech services between GSM and IP-based wireless local area networks.