One distinct possibility is the fact that the wi-fi phone is still registered to Voxalot while conducting this test. I have seen this situation in the past, where there are no SIP devices currently powered up/running that can answer a call, but they are still within the period specified by the registration interval, and instead of getting voice mail (as would happen if no devices were registered), you get instead a busy signal. It clears up though as soon as registrations expire.
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