If the tone returned is detected as busy or the adapter detects ring back tone and the telephone does not answer after, in one example, three rings , the adapter will hold the DID line up to {TAIL ~ the Central Office} or E&M tie line to {HEAD ~ the originating switch} 
If the tone returned is detected as busy or the adapter detects ring back tone and the telephone does not answer after , in one example , three rings , the adapter will hold the DID line up to { TAIL ~ the Central Office } or E&M tie line to { HEAD ~ the originating switch }
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