I have one SIP trunk (DrayTEL) which is the default trunk for all of my extensions. I have digit map entries in the dial plan to force certain calls over PSTN1.
I am registering an extension via CSipSimple running on Android 7.1.2. The PSTN calls work perfectly (allowing for the ropey audio quality.) The SIP calls work perfectly to the DrayTEL service numbers (e.g. speaking clock) and other addresses on the DrayTEL network but not to any other SIP address (e.g. music@iptel.org.) Attempts to dial these addresses result in CSipSimple reporting "400 / Bad Request". Dialling the failed addresses when registered to thee DrayTEL account directly on the phone rather than via the VigorBX is successful so I suspect the problem is with the BX. I have the latest 3.8.8.4 firmware installed.
Does anybody have any idea what I might do to try to fix this problem?

Further information: Using Linphone instead of CSipSimple does allow calls to non-DrayTEL addresses but does not allow calls to DrayTEL addresses.

Well where's the facepalm emoji? I just looked at the DrayTEL website and saw this: "VoIP to VoIP calls are free within the DrayTEL network." Clearly I need to spend money to break out of their network.
Not sure how the CSipSimple/Linphone discrepancy is explained though.

Teh PBX should be able to call ANY SIP address FOC - those don't need to go via DrayTEL (or any SIP ITSP)..

Then clearly either:

a) I have the PBX configured incorrectly

b) I have the handset software configured incorrectly

c) The PBX hardware or software is faulty

d) The handset hardware or software is faulty

I'm not sure how to diagnose the problem.

I've added a couple of other SIP trunks from different providers and what I've found is that any call to 'user@provider' will be correctly routed if I have a 'provider' trunk available. If I don't have a trunk from a particular provider then I can't call any user on that provider's network.