internal mobile number transfer

Hi, I was wondering if you could suggest and help me with transferring my mobile number from one ID mobile plan to another. My sim-only plan for 1 month has ended. I believe instead of upgrading my plan, I have taken out a new sim-only plan altogether. This has now meant that I will be getting a new mobile number and this is not what I intended. Is there any way to keep my original phone number? I still have the same phone also. Thank you.