Presumably you have registered all your Orbtalk accounts on Voxalot? If just one isn't working, have you tried deleting it and reinstalling/registering?
Alternatively, how about setting it to divert to
xxxxxx@voxalot.com in Orbtalk's control panel? (I presume Orbtalk allows you to forward to a SIP URI?)
Another alternative with "errant" DIDs is to register them in MySipSwitch or PBXes and set the redirection to Voxalot there. That's how you handle Sipgate DIDs (which won't register directly on Voxalot) for example.