New Direct Routing setup - Couldnt complete call

we are setting up a new implementation of Teams Voice Direct Routing using an existing SBC being used for on premises Skype for Business. so far with a test user all works OK apart from making external calls, when attempting this from the Teams client we get a "Declined" error, if I try from a web client i get the following

The test user has an A5 license and we have looked over all the settings in teams and done the usual searches but have not come up with anything of use. any pointers please.