Rental vacting notice not received by agent?

we faxed a notice to vacate our rental property which is stated you must give 21 days, 2 weeks later we called the agent to see how much rent we owe, they are now telling us they did not receive the fax and we need to start over and give notice from that date even though we have given proof that the fax was sent they are claiming it is not satisfactoy. what atre our rights here as in no wasy does it state you must call the agent after faxing the notice to vacate.