Yes, if you work with different departments in Teamleader, you can set a default department. 

To do this, follow these steps:

  1. Go to Settings > Company information
  2. Click on the pencil next to the department you'd like to set as default department
  3. Click on Edit next to "Filling out your company's info (name, address, email,..)
  4. In the General information, tick the box next to 'Default'

This will cause this department to be the default one, and therefore the one used by default when creating invoices, quotations or when using a signature when sending an email. Remember that even if this is marked as default, you can continue to manually select the other departments, when necessary.