You might use Raspberry Pi as Ethernet shield, and load Asterisk at Raspberry Pi, and connect between Raspberry Pi and Arduino via usb cable.
Asterisk support;-
AGI (Asterisk Gateway Interface)
IVR (Interactive Voice Response)
TTS (Text-to-Speech, a lot of languages supported)
...
say Arduino has relay and temperature sensor, the person could call the Asterisk phone number, go through IVR, get temperature via TTS, and switch on/off relay.
or Arduino push temperature reading to Asterisk AGI, and once AGI preset temperature reached, it will generate call.file and trigger asterisk make outbound call to play back prerecord sound or TTS voice.
Arduino+Asterisk could do much more, a little list here.