Asterisk 1.8.7 SIP Channel Choppy

I have Asterisk 1.8.7 setup on Centos 6.0. I could not figure why I’m getting choppy sound from SIP channel. I using to different SIP providers to test and the result is the same. I have “internal_timing=yes” in the asterisk.conf. There is no hardware timing source. Currently, I only have one SIP phone setup which is a Grandstream GXP1450. If I call the PBX from my cell phone, the ring tone is kind of choppy and the voicemail prompt is choppy too.

Normally, I can hear the incoming call clearly; however, the other end said the sound is choppy.

I’m wondering what I missed in the configuration. Any suggestion on troubleshoot this issue will be greatly appreciated.