Can I block a phone number not on my contacts

Yes, if that number has called you. Open the Phone app, go to Recents, tap the blue i in a circle to the right of the phone number, tap Block this Caller.

Another option is to silence all callers that are not in your Contacts. See the "Silence Unknown Callers" option in Detect and block spam phone calls - Apple Support

If they have only texted you, go to their message, click on the big circle at the top (like you would view their contact information) click on the info button on the far right, scroll all the way to the bottom on the contact and click Block Caller

If they have called you, go to recent calls, click the blue circle (on the far right) with the “i”, scroll down to the RED block caller option

Yes you can block a number not in your contacts