Defaultexpiry < 230

So far, my asterisk running on my Fonera WiFi router works with only one problem. I have to set defaultexpiry=180 (maximum to 230 seconds) on the sip.conf file; otherwise, incoming calls from FWD will hit FWD voicemail and the CLI (with verbose=20) will show no activity of any FWD incoming calls. If I lower defaultexpiry=180, my asterisk won’t have problem to receive any FWD incoming calls. I sure would like to set my asterisk with a much higher defaultexpiry value, i.e. 1800 seconds. Does anyone have a solution here?