This paper describes a reference architecture which enables a H.323 user, to get access to H.323 based Multimedia services also outside her/his corporate network domain via an IMT2000 based mobile network. The appropriate mechanisms to handle the mobility management between an H.323 based environment and IMT2000 based mobile networks (combined mobility) are explained. The article also considers the appropriate requirements to achieve defined conditions for an end to end service with a guaranteed quality. Furthermore the demand of corresponding Service Level Agreements between all involved parties is identified.