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One problem that often occurs after installing/implementing an IP telephony system is voice quality. Although there are objective measurement tools for voice quality evaluation, the prices of these are very expensive. Therefore, back to basics, this paper focuses on subjective tests. This study used the data from three tests, consisting of listening-opinion tests, conversational tests and interview tests. All were conducted using the same IP telephony system with G. 711 codec. All tests, following the ITU-T P. 800, were in the best condition. The subjects who participated in the tests were 163 students and 1 worker in King Mongkut’s University of Technology North Bangkok (KMUTNB). This study compared and analyzed the data from 3 kinds of subjective tests using ANOVA, to see if the data from the interview tests were consistent significantly with the data from the listening and conversational-opinion tests. The results, called the Mean Opinion Score (MOS) values, from the interview, conversational, and listening-opinion tests were 4.14, 4.16 and 4.23 respectively. Also, the analyzed result shows a p-value of 0.511. This means the MOS values from these three methods are not significantly different. Therefore interview tests can be used to evaluate voice quality, and is as good as other subjective methods, without high cost of expensive tools, making it very applicable in developing countries.