The PCM files for the rings must meet the following requirements for proper playback on Cisco IP Phones:
• Raw PCM (no header)
• 8000 samples per second
• 8 bits per sample
• uLaw compression
• Maximum ring size — 16080 samples
• Minimum ring size — 240 samples
• Number of samples in the ring must be evenly divisible by 240
• Ring should start and end at the zero crossing
You can use any standard audio editing packages that supports these file format requirements to create PCM files for custom phone rings.