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This article proposes a solution for two issues in current communications. Firstly, that IMS suffers from client inexistence. Secondly, that mobile operators want to give the subscriber a possibility to access their VoIP network and efficiently cover special areas like airports. To address these problems, we have developed a novel service architecture, which allows 2G subscribers access to a SIP based VoIP network via WiFi complying security standards. User authentication and authorization is based on the EAP- SIM algorithm. Integrity and confidentiality is provided by IPSec connection established using parameters derived from authentication triplets. Consequently, all security-related issues can be performed exclusively using the subscriber identity module (SIM). We have tested and verified the proposed architecture with a mobile phone and have proven the correctness of our approach. The main drawback that remains is the difficulty of IPSec implementation that can be bypassed by a special application.