asterisk users - Oct 2013 - Phones flashing but not ringing

We have been using Asterisk SLA for a while with Cisco SPA series phones. Once
in a while the phones flash, but not ring when a call comes in. We can pick it
up and talk to the caller even if that's the case.

This is pretty random (might not happen for couple of weeks). The quick solution
is to restart Asterisk which we are trying to avoid. What might cause this?

Thanks,
Matt
 		 	   		  
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Have you tried restarting the phone instead of Asterisk? I don't think that
Asterisk sends
separate commands to the bell and to the call LED. Since the LED is flashing, it
is likely that
the "SIP INVITE" signal from Asterisk is ok. Also the ring tone
normally does not come from
Asterisk itself.

jg