How to unblock 3,500 numbers on iPhone?

On the iPhone, it is one at a time. If you have a Mac, you can bulk delete those contacts there and it will sync to any iPhone using the same Apple Account.

You can send Feedback to Apple here for a feature request to add the same bulk delete option on the iPhone that is available on the Mac.

Feedback - iPhone - Apple

On the Mac, first you have to make sure the Apple Account signed in is the one on your iPhone. You do that by going to Settings > [name on top], and if that is not the Apple Account you want to use, you can sign out, then in with the correct Apple Account for your phone.

To bulk delete the Contacts on your Mac it does depend on which Mac OS version you are using, but you will find those Contacts in either of these places:

•  > System Settings > Privacy & Security > Blocked Contacts

or

• Open the Mail app and choose Settings in the Mail menu. After that choose Junk Mail and then select Blocked Contacts.

No matter which method you use on your Mac, you can bulk delete them by selecting the first Contact, then Shift-Click the last one to select all those within that range. After that, click the minus button on that bottom of that page. Give it a minute or two to sync those to your iPhone and you can verify that they have been removed on that device. After that if you were using a different Apple Account on the Mac, you can sign back in with that one.

How do I do that on a Mac? I have no idea how to use a Mac or how to find my phone on it. My husband has a Mac that I could use but I have no clue what to do and neither does he.