Time varying phase noise is a limiting factor in highspeed wireless communication systems, e.g., microwave backhaul links in cellular networks. This paper seeks to investigate the performance of circular M-ary Quadrature Amplitude Modulations (M-QAMs) in the presence of time varying phase noise. A new approximate union bound expression for the Symbol-Error Probability (SEP) of a specific circular M-QAM constellation is derived. Numerical results show that this expression is accurate at medium-to-high Signal-to-Noise Ratios (SNRs) for different constellation orders, M. Next, exact closed-form expressions for the SEP of Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK) modulations in the presence of phase noise are derived.