Problem about chan_ss7

I have installed chan_ss7-0.8.4 in my two asterisks and connected back to back. I can make call each other using iax phone. But the system always prompt write buffer full error as follow when i make call.

PE860B*CLI> Jan 24 14:10:04 NOTICE[9930]: chan_ss7.c:2017 ss7_write: Write buffer full on CIC=30 (wrote only 0 of 320), audio lost.

Why? who can help me? Thanks