With 30 concurrent calls, at a rate of 10 / second, we get this:

Jun  8 10:09:13 ERROR[8315]: chan_sip.c:11370 sipsock_read: We could NOT get the channel lock for SIP/sipp-bba9! 
Jun  8 10:09:13 ERROR[8315]: chan_sip.c:11371 sipsock_read: SIP MESSAGE JUST IGNORED: ACK 
Jun  8 10:09:13 ERROR[8315]: chan_sip.c:11372 sipsock_read: BAD! BAD! BAD!
Jun  8 10:09:23 ERROR[8315]: chan_sip.c:11370 sipsock_read: We could NOT get the channel lock for SIP/sipp-21b2! 
Jun  8 10:09:23 ERROR[8315]: chan_sip.c:11371 sipsock_read: SIP MESSAGE JUST IGNORED: ACK 
Jun  8 10:09:23 ERROR[8315]: chan_sip.c:11372 sipsock_read: BAD! BAD! BAD!

Also, when we hit 335 calls, we start to get open file issues:

Jun  8 10:05:06 ERROR[28006]: rtp.c:945 ast_rtp_new_with_bindaddr: Unable to allocate socket: Too many open files
Jun  8 10:05:06 WARNING[28006]: chan_sip.c:3101 sip_alloc: Unable to create RTP audio  session: Too many open files
Jun  8 10:05:06 WARNING[28006]: acl.c:306 ast_ouraddrfor: Cannot create socket

I've jacked up the open file limit for the kernel w/ sysctl..So I'm doing something wrong..