If your MicroSIP stops playing the ringtone, users have reported that a clean exit and restart of the application often resolves audio glitches where calls come in but no "ringing" sound is heard. MicroSIP Downloads - Installer and Portable version

Your ringtone is too loud, causing clipping. MicroSIP does not have a built-in limiter. If your WAV file was normalized to 0dB, it will sound distorted. Use Audacity to reduce the gain to -3dB before saving. A clean, hot tone is better than a distorted, messy one.

Free for commercial use, highly stable, supports multiple SIP accounts, and consumes near-zero system resources.