The queue still ring and identified the agent is Not in use. But this agent is offline(unkown/UNREACHABLE)

I have Sip ext range 30X (301 --> 309) . But i just got this problem with ext 309 . Run queue show and i found 309 is not online but the queue still show Not in use for 309. Then i put out another ext nomal , just have 309 is queue and that issue happening.
i try to remove the ext 309 (SIp) and recreate again. But still same sittuation.