Problem with asterisk embedded

I make an asterisk box usign an wrt54gs with open wrt and asterisk 1.2.1
I make the configuration and all seems working.
The problem is that i must reboot the asterisk boot because it seems to be immpossible to receive more than one call without rebooting…From the shell i get " WARNING[669]: chan_sip.c:3414 process_sdp: Insufficient information for SDP (m = ‘’, c = ‘’)

What does it means? What can i do?