No RTP ports remaining Can't setup media stream for this cal

I’m running asterisk in VMWare, and due to this issue, I have to manually input some RTP ports mapping in the VMWare network console, I mapped 25 ports for RTP.

After some times, there is an error:

[Mar 28 23:24:17] ERROR[9140]: rtp.c:2517 ast_rtp_new_with_bindaddr: No RTP ports remaining. Can't setup media stream for this call. [Mar 28 23:24:17] WARNING[9140]: chan_sip.c:6272 sip_alloc: Unable to create RTP audio session: Address already in use

I find that the asterisk didn’t release some RTP port by running "netstat -a"
there are originally 6 ports using, when 1 sip user called to another sip user, then there are 10 sip ports using, after a period of time, there are still 8 ports being used… why? would asterisk those 8 ports after some times? i set the rtptimeout=30 in sip.conf, and it didn’t help.

(Due to the configuration problem of VMWare, i cannot manually input 10000 port mapping in the network console, so i just use 25 ports…)

Version using:

lamsoft-pbx*CLI> core show version
Asterisk built by root @ lamsoft-pbx on a i686 running Linux on 2009-03-26 02:08:44 UTC

Thank you.