A system for an adaptive network phone includes a SIP-enabled handset or other telephone device to be connected to a host computer, for instance via a USB connection. The host computer may initiate a call event when the user picks up the SIP device and wishes to make a call, in one regard by testing line conditions for a data connection to the Internet or other network to determine if a packet-based call will meet minimum noise, delay or other criteria of the user. If the conditions are satisfactory, the call may proceed as a packet network transmission, otherwise the call may revert to the public switched telephone network for fallback completion. The SIP-enabled handset may also interface to multimedia functions in the host computer, for instance to permit paging, answering machine, cordless operation or other features.