no sound for incoming calls after a few days

I have configured asterisk server with a few IP phones and they are all assigned with static IP addresses under same subnet mask.

I use a Switch to inter-connect them and there is no router / broadband internet for this setup.

Initially it works well with incoming / outgoing calls through analog lines.

after a few days incoming calls could not pickup (there is no sound for both parties when pickup) from the phones although i actually can see the incoming call from that particular phones. Rebooting of the phone does not solve this problem. However I still could use this phone to make outgoing call.

anyone knows roughly what's the problem? Do I have to use a router or must have the broadband internet for the setup?

thanks for any advice.