How to make asterisk use new nonce for re-register

Hi, all

I’m trying to make an Asterisk trunk register with a Kamailio proxy server using the register => line in sip.conf. The initial register is ok. But after several re-register , the Kamailio server rejects the trunk due to expired nonce. Looks like Asterisk UA keeps reusing the same nonce forever for re-register. This seems to defeat the purpose of using nonce to make replay attack harder. Wonder if there is a way to make Asterisk generate a new nonce for each re-register.