Setting Up Security Filters Within Roles

Hi Forum, please can somebody advise on a problem I have with Security Filter within Role. Let me explain, my client has sales reps and wants the reps to only see their own Customers and Contacts so I firstly created a Security Role for their functional area and then applied some Security Filters to certain tables namely Customer and Contact. The problem I have is this, when they go to a Customer and then press the Sales button to look at existing orders, they get a message telling them that they do not have permision to read the Contact table if an order exists. To get around it I removed the filter but now they can see all Contacts. Also, if they create a new Contact and then try to Create it as a Customer, they get a message telling they do not have insert permission on the Customer table so I have to remove the Security Filter on the Customer table. The net result is they can now see all customers. I’m going around in a big circle here from filtering so they can only see their own Customers and Contact to allowing them to see all because they cant create Customers from Contact and cant see Orders on Customers. Can somebody please either explain what I’m doing wrong or suggest an alternative way of doing the filtering. The Navision version is 4.0 and NOT SP1. Thanks…Paul