Your file is rejected by 3CX?
Cause 1: an MP3 renamed to .wav
By far the most common. Renaming prompt.mp3 to prompt.wav does not change its contents: the bytes are still those of an MP3, and 3CX rejects it. The checker reads the real bytes of the file, never its extension.
Cause 2: wrong sample rate
44.1 kHz or 48 kHz are the defaults of every recording tool. 3CX expects 8,000 Hz. 16 kHz is rejected too on a standard system.
Cause 3: stereo or 24-bit
Telephony is mono and 16-bit. A stereo file, or a 24-bit one, is rejected even when its sample rate is right.
Cause 4: a WAV that is not PCM
A .wav container can wrap A-law, u-law or floating-point PCM. 3CX expects uncompressed integer PCM (format 0x0001).
Cause 5: the file imports, but nothing is heard
The format is right, yet the prompt is inaudible or distorted. Two causes: a recorded level far too low, or several seconds of silence at the start — the caller then assumes the line is dead and hangs up.