Your handsets run across our internal network, which is very robust, and out on to the internet via our 500MB leased line. We try to provide the best service possible at all times, but like most providers it is impossible to guarantee a 100% 24/7 permanent connection. Internet telephony (SIP) relies on multiple connections across the internet, any of which may fail and is beyond our control. Our service level agreement gives more detail. If your business needs this level of service, then the installation of a separate backup telephone line is recommended.