Namespace quirk

We have some namespaces implemented from a supplier's code - these have a namespace like SupplierName.Core.BusinessLogic and SupplierName.Entities.ReturnArguments and we are writing our own company code to fit in with this.  We are using MyCompany.SupplierName... as a namespace to segregate our code relating to their program from the rest of our applications' code (each has its own namespace).  When we include their dll in our code their root namespace seems to conflict with our second-level namespace - if we try to call SupplierName.Entities.ReturnArguments it fails as it seems to be trying to look in our MyCompany.SupplierName namespace rather than working from the root.  However if use "using SupplierName.Entities;" in the header this is fine.  Is this correct that it would look in our class's root namespace for SUpplierName first, rather than looking for a root namespace called SupplierName - this ...