I hope I'm not contributing to some new urban legend. I ran this by one of my engineers and he provided a possible explanation.
Most house phones are analog in nature and provide an audio range of 3 or 4khz -- max range. As such, they probably can't transmit at the frequency range necessary to trigger the auto unlock feature. The exception to the house phone would be the house phone using ATT digital service, Comcast phone service, vonage and the like. In this case the phone is actually using a digital data network for transmission (VOIP). He thinks that this feature is triggered by frequency not sound. Even an analog phone is capable of frequence distribution. The bigger question is one of range.
On the other hand many mobile phone providers are running on digital networks capable of transmitting on a wider frequency range. I would suggest that when you test, see if you are on a digital network. Try the same test when you are roaming on a non-digital, analog network. This may help isolate the possibilities....
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