In that case, the existing SIP trunk would remain as is. You are simply having an additional DID (the ported number) added for incoming. You will have to create a DID rule if you don't already have one. for outgoing the existing trunk will either "send" the number defined in that trunk settings, or as set against each extension (outbound caller ID). Changing that in the trunk settings may have consequences, as I assume that everything else, to do with the existing trunk is to remain in place. You may have to contact your provider if you experience problems.