I recently tried to install a Viking e-30 door phone on a Cisco FXS port. Everything worked fine except the door phone drops the call after about 3-5 secs. I spoke with Viking's tech support and they advised me that the issue was caused by low voltage. The solution they provided me with is to purchase a 12v adapter from radio shack and wire it in series with the door phone. I'm not so sure about this solution and am interested to hear if anyone else has tried this before?