To make such a test you need to have

1. 2 SIP registrations created in Voximplant - one for each extension provided by PBX vendor.
2. 2 scenarios in Voximplant. One of them should make outbound call using SIP registration related to the first PBX extension using VoxEngine.callSIP with regId parameter specified Second scenario will receive calls from the second extension. For that you need link SIP registration to a routing rule containing the desired scenario.

More information can be found here