Carrier-envelope phase (CEP) of single-cycle pulses generated through two-color filamentation has been investigated. We have observed a particular behavior of the phase: the phase of high-frequency components of the generated pulses changes continuously and linearly with the relative phase between the two-color input pulses, whereas the phase of the low-frequency components takes only two discrete values. The transition of the phase behavior has been clearly observed by using frequency-resolved optical gating capable of CEP determination. We have found out that such a phase behavior is a unique feature of single-cycle pulses generated with a passive CEP stabilization scheme. (C) 2015 Optical Society of America