Outbound caller ID is handled differently in 3CX v15 than in previous versions. Before, outgoing calls would automatically use the extension's caller ID, whereas this new version, it will automatically use your SIPTRUNK.com Trunk #. To fix this, it is necessary to update an outbound parameter. Without this small step, your Trunk # will show up as your caller ID on all outbound calls.
To change this, first ensure at least one of the extensions that has its own caller ID has a proper value set:
Then, go to the SIP Trunks tab on the left toolbar, double-click on your SIP.us trunk, then select Outbound Parameters from the top toolbar.
Finally, change the dropdown value for the From: User Part field to "OutboundCallerId" Outbound caller Id taken from Extension settings in management console. Once you do this, all outbound calls will take on the caller ID value for the originating extension.