Full Error Message:
Insufficient Privileges: You do not have the level of of access necessary to perform the operation you requested.   Please contact the owner of the record or your administrator if access is necessary.

Solution:

Salesforce users with restricted permissions on their profile  may see this error message if they don’t have proper access to A5 Fax app package components. For example, if they try to access “send fax” VisualForce page that comes with installed package and that user’s particular profile does not have access to this VisualForce page, they could see this error message.

You can review all app package components that users may need access to by going to Setup > Installed Packages > click on A5 Fax app > click “View Components.” Also, review the Set User Permissions section of the Basic Configuration article.