Nigeria with infant GSM mobile subscribers is mostly concerned about the quality of services they received from the GSM operators in the country. Nigeria is also concerned about the factors affecting voice quality such as degradation of speech (voice), messages and other anomalies encountered. The paper focuses on the evaluation of voice quality of some GSM operators in the country. It looks into seven items used to measure the dimension of voice quality such as echo of the sender's voice, distortion in the receiver's voice, volume of the receiver's voice and overall voice quality perception. In this paper a survey design was used. The sample consists of 300 subscribers and these were selected by stratified random sampling technique. Questionnaire containing twenty six (26) items on factors affecting voice quality level in mobile communication was administered to the various categories of subscribers of mobile communication in Oyo State, Nigeria. Mean and standard deviation was used to answer six research questions. Results show that the highest overall mean opinion score is 3.10 and the least value is 2.93. It therefore implies that the voice quality levels are above average for the four GSM service providers under investigation.