The transition from a cellular data network to a WiFi network configured to use a proxy can sometimes take several minutes and results in proxy not found errors in the Lync application until the transition is complete. The issue occurs when the WiFi network is configured with higher priority than the cellular network connection and it has only been observed with selected service providers.

If no action is taken, the transition will eventually occur and connectivity to the WiFi network will be established.

Alternatively, you can log out of Lync and log in again with the WiFi connection.