How to fix N95 SIP?

Well, currently, I'm also using N95 SIP with Asterisk (v 1.2.14 though), and it works fine here.

In your case, it may be happening due to some error in wifi disconnects with earlier N95 firmware. Mine is currently on 12.0.x. you can check your message box for any error message. At any time my phone goes beyond wifi range, I see the following in /var/log/asterisk/messages:

NOTICE [3550] chan_sip.c: Peer '14' is now UNREACHABLE!

You can connect other phones from asterisk and It would help if you post relevant parts of your asterisk configuration, as well as your SIP settings from your N95. Otherwise you can check for these things:

Examine about a NAT (network address translator) or a firewall on the route.

If Asterisk supports TCP transport, using it may help, if Asterisk is configured to be the outbound proxy, change the transport in the proxy settings to TCP instead of UDP or Auto (when UDP is used for SIP requests below 1300b)

The WLAN is not woken up from power save - what is your WLAN access point vendor and type? If there is selection to disable power saving features it might help.

If the problem is not resolved by this, then you can take a look at this link: www.developer.nokia.com.

Best of luck!