If a client rings up about booking a job, you must ask them:
"Is that for a new job or a continuation?"
Use "Job Maintenance" to find the job in which the client wants to continue. Left-hand click the job once and the pres the "Continue Job" button circled in red.

Then book the job like normal. Still ask for an order number, because some continuing jobs having different order numbers every month.
**DO NOT CONTINUE JOBS IF THEY ARE OLDER THAN ONE WEEK**
use "Rebook as New Job" instead