SIP phone - timeout?

Using asterisk @ home (Asterisk ) and a SIP 841 phone.

The sip phone is on a cable modem and uses a 192.168 addy.

When the sip phone is first powered on, it can make and receive calls without a problem.

After a few minutes, the sip phone doesn’t ring when called. The asterisk server sees the phone being called.

If the sip phone makes an outbound call, then after the call, the phone can receive calls for a few minutes.

Any suggestions where to look next?