| 摘要 |
repeatedly detecting the supervisory audio tone; calculating an arithmetic average of the repeatedly detected supervisory audio tone values; checking whether the arithmetic average is within a corresponding frequency; and checking a call state of the searched frequency range, if the designated supervisory audio tone is not detected for one or more hour, muting a call, and if the designated supervisory audio tone is not detected for two hoursor more, forcedly completing the call, thereby relatively enlarging a call range.
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