We had the exact same issue with a client of ours. We were using mobile apps as our sip clients, and they would just stop working, along with the ESNA. Tech support stated that the phones were not supported, and that they were registering too often and overloading the sip stack, which then you have to shut the system down to restart it. We backed the registration down severely and it still happened. We never got a resolution either, and ended up having to put digital handsets everywhere. Seems like the Iwatsu system does not like playing well with anything sip besides polycom soundstations (limited) and ESNA voicemails in my opinion.