Can I re-register my iphone number if I deregister it and is any data lost?

Simply go to Settings > Messages > iMessage > [Enable]

If you get an error when trying to activate iMessage or FaceTime

To use iMessage or FaceTime, you need to activate them on your iPhone, iPad, or iPod touch. If you see an error message during activation, follow these steps.

When activating iMessage or FaceTime, you might see one of these messages:

• Waiting for activation
• Activation unsuccessful
• An error occurred during activation
• Could not sign in, please check your network connection

Check your device settings

1. Make sure that you’re connected to a cellular data or Wi-Fi network. If you're using an iPhone, you need SMS messaging to activate your phone number with iMessage and FaceTime. Depending on your carrier, you might be charged for this SMS.
2. Go to Settings > General > Date & Time and make sure that your time zone is set correctly.

Turn off and restart iMessage and FaceTime

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device.
4. Turn iMessage and FaceTime back on.

Still not activated after a day?

It might take up to 24 hours to activate iMessage and FaceTime. If the issue continues after 24 hours, follow these steps:

1. Make sure that your device has the latest version of iOS.
2. If you're using an iPhone, contact your carrier to make sure that you can receive SMS messages.
3. Contact Apple Support.

If you get an error when trying to activate iMessage or FaceTime ...

https://support.apple.com/en-in/HT201422

Annabees wrote:

Can I re-register my iphone number if I deregister it and is any data lost? One phone is a work phone 6S and one is personal 10 and I gave the 6s to a staff member but they are getting personal message from ppl with iphones.

It sounds as if you're using the same Apple ID on each phone. You shouldn't be sharing your Apple ID with another person. This is just one of the things that can happen. They should get their own Apple ID and have their own data.

Sign out of iCloud and the iTunes & App Store. 

If you're using iOS 10.3 or later, tap Settings > [your name]. Scroll down and tap Sign Out. Enter your Apple ID password and tap Turn Off.

If you're using iOS 10.2 or earlier, tap Settings > iCloud > Sign Out. Tap Sign Out again, then tap Delete from My [device] and enter your Apple ID password. Then go to Settings > iTunes & App Store > Apple ID > Sign Out.

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support