Artifact on playing a audio file on a phone from a Asterisk server

I have an Asterisk SIP server. When I playback an audio file (.ulaw file, compressed using ulaw) I hear a noticeable click (or sound artifact) before the playback begins. This "click" is not in the actual audio file and happens at the start of every Playback command in the ael script. Should I be using a different format, is this a codec issue开发者_如何学运维, how do I resolve this issue?

Here are some of my files:

http://kscserver.com/hello.zip

http://kscserver.com/thankyou.zip

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Without looking at the file, it's hard to say, but if the first sample of the file starts at some value other than 0, you may get a click (since the output will go from 0 to N in one sample - a broad noise impulse). If you don't know a sample starts "clean" it can make sense to ramp it in volume-wise, or search the uncompressed data for a zero-crossing and start there.