[Asterisk-Users] CISCO 7960 Goes missing

[Matt]

I've got a number (10) Cisco 7960's connected to my network.  All the phones
work perfectly except one.

The asterisk console keeps throwing up:
Jun 22 15:39:15 NOTICE[-1147470928]: chan_sip.c:5887 sip_poke_noanswer: Peer
'4001' is now UNREACHABLE!
Jun 22 15:39:27 NOTICE[-1147470928]: chan_sip.c:4925 handle_response: Peer
'4001' is now REACHABLE!
Jun 22 15:42:08 NOTICE[-1147470928]: chan_sip.c:4925 handle_response: Peer
'4001' is now REACHABLE!
Jun 22 15:43:12 NOTICE[-1147470928]: chan_sip.c:5887 sip_poke_noanswer: Peer
'4001' is now UNREACHABLE!
Jun 22 15:43:36 NOTICE[-1147470928]: chan_sip.c:4925 handle_response: Peer
'4001' is now REACHABLE!

I've checked the cable and even swapped out the phone but 4001 is always
disappearing off of the network.

Anyone got any hints?

Obviously I've added qualify=yes to my sip.conf in an attempt to
troubleshoot this but I'm now getting nowhere fast!

Matt