Swap SIM for number portability

Simply activate iMessage on your Old SIM

To activate iMessage and FaceTime using your Phone number

First, check to know whether you have an SMS plan enabled at least for One SMS*** with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS*** Only)

If you can't turn on or sign in to iMessage or FaceTime - Apple ...

*** You may not have an SMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS services.

Why do you want to de-register? de-register is only done when you want to use the same SIM on a non-iOS device.

If you still have an iPhone and want to stop iMessages on that number simply go to

Settings > Messages > iMessage [Disable]

Settings > FaceTime > FaceTime [Disable]

That's it.

Probably you are wanting to remove this old number from Apple ID. If that is so watch the GIF below