Problem with Formula (text) field.

Hey Everyone,

I have created a formula field to calculate when leads require a phone call. Basically once a leads responds to a campaign, they require a call. An scontrol automatically populates the date field for Last_Call__c, then the formula below will compare the fields to see if a call is required. The formula works fine except when the Last_Call__c field is blank. I added the second line of code to try and fix this. It should look to see if the Last_Call__c field is blank and the Last_Campaign_Date__c is not, then show a "No call since last campaign response" alert. It doesnt give an error however the logic doesnt work correctly. Any ideas on what I can do?

IF( Last_Conversation__c < Last_Campaign_Date__c , "No conversation since last campaign response",
IF ( AND( Last_Call__c = null, Last_Campaign_Date__c <> null), "No call since last campaign response",
IF( Last_Call__c < Last_Campaign_Date__c, "No call since last campaign response",null
)
)
)

Thanks,
Rich