'Got SUBSCRIBE' message

What does this mean? And how can I go about trying to fix this.

NOTICE[11348]: chan_sip.c:14781 handle_request_subscribe: Got SUBSCRIBE for extension 2400@sip from, but there is no hint for that extension

Sip clients must subscribe to the Asterisk server, or they cannot use it’s services.

Subscribe messages are normal. A sip client will renew it’s subscription periodically. (Usually every 3600 seconds.)