Territory Description join field value

Hi

we want to bring the description field that is in Territory through the opportunity.

I have tried the following but no joy.  This works for us on the account level but does not seem to work on the opportunity level.  I Don't really want territory and description but thought I would try this first.

JoinFieldValue('<Territory>',[<TerritoryId>],'<Description>')

Has anyone any ideas how I can get territory description in the Opportunity?

Thanks

Paul