When one phone has a dial tone, but two phones do not, what is the most likely cause?

The situation where one phone has a dial tone while two others do not suggests an issue with the connectivity or wiring between the phones. An open circuit in the wiring is a likely cause as it indicates that there is a break in the electrical path that would allow the phones to receive a signal. This would result in the functional phone being isolated from the others, which still may be connected to the same phone line but unable to complete the circuit due to a wiring issue.

In contrast, the other options offer explanations that do not align well with the symptoms described. A power failure to the outlet would generally affect all phones if they are reliant on external power, assuming they are corded and not battery-operated devices. The incorrect phone model does not usually prevent a dial tone; it might cause compatibility issues but wouldn't selectively allow one device to function. Faulty telephone connections would typically affect all connected devices rather than creating a situation where only one phone works as indicated. Thus, considering the specific scenario described, an open circuit becomes the most logical and accurate explanation.