[VoiceOps] Asterisk 1.8.6.0 unable to allocate a port for RTP instance

Hi list! On the first we upgraded one of our Asterisk boxes from 1.4 to 1.8.6.0 everything ran OK until tonight we started getting the following messages on the asterisk console and in the log file. [Sep 15 18:05:21] ERROR[14306] res_rtp_asterisk.c: Oh dear... we couldn't allocate a port for RTP instance '0xb138$ [Sep 15 18:05:21] ERROR[14306] chan_sip.c: Got SDP but have no RTP session allocated. Looked at the active number of channels using core show channels and no channels were reported as being in use/active. had to restart the asterisk service to restore access. No other messages in the log to indicate what could be the issue. Anyone else ever see that happen? Thanks! Joseph