  For short-packet, low-latency communications over random access channels,
piloting overhead significantly reduces spectral efficiency. Therefore,
pilotless systems recently gained attraction. While blind phase estimation
algorithms such as Viterbi-Viterbi Phase Estimation (VVPE) can correct a phase
offset using only payload symbols, a phase ambiguity remains. We first show
that the remaining phase rotations in a polar coded quadrature amplitude
modulation (QAM) transmission with gray labeling are combinations of bit-flips
and automorphisms. Therefore, the decoder is equivariant to such phase
rotations and, by smartly selecting the frozen bits, one can jointly decode and
resolve the phase ambiguity, without the need for pilot symbols or an outer
code. Our proposed system outperforms pilot-assisted transmissions by up to 0.8
dB and 2 dB for quaternary phase shift keying (QPSK) and 16-QAM, respectively.
