Test the hangup cause, and if it suggests a network problem, repeat the dial via the other ITSP.
Note that configuratons where there are multiple, disjoint, networks seem to cause problems, although Asterisk does try to work out the interface address to use in SIP headers.
Configure a route to their address via the interface. That is is an OS configuration issue, basically the same as configuring routes to non-local subnets on your LAN.