An Internet protocol (IP) telephony system terminates calls to certain groups of telephone numbers via multiple different telephony carriers. The IP telephony system will discontinue using a telephony carrier to terminate calls if the quality provided by the carrier falls below a threshold level. The IP telephony system includes a quality monitoring unit that determines when a particular group of telephone numbers are intrinsically impaired, such that no carrier could provide high quality when terminating calls to those numbers. In these circumstances, the IP telephony system adjusts the quality threshold to which a carrier's quality is compared when completing calls to the impaired numbers to account for the intrinsic impairment of the telephone numbers.