What do I do when someone is using my phone number to make spam calls? Strangers keep calling me saying they are returning a call with my number.

If you want to spend the time and effort, you could report it to the FCC, but then the question becomes would the FCC spend the time and effort.  I'm sure the FCC will not trace who's doing this.

Nothing was hijacked.  Someone is only "spoofing" your telephone number from somewhere else.  This someone didn't specifically select your number but selected a number...it just happens to be yours.

Normally, "spoofers" just pick a random number and then move on to another number, so it's weird you've been getting calls all week, unless it has been taking a few days for these people to get around to reviewing their missed calls.

Ride it out for a few days.  I'm sure it'll stop.

From the FCC website (Q & A) about spoofing:

What can you do if your number is being spoofed?

If you get calls from people saying your number is showing up on their caller ID, it's likely that your number has been spoofed.  We suggest first that you do not answer any calls from unknown numbers, but if you do, explain that your telephone number is being spoofed and that you did not actually make any calls.  You can also place a message on your voicemail letting callers know that your number is being spoofed.  Usually, scammers switch numbers frequently.  It is likely that within hours they will no longer be using your number.

So...the FCC will basically do nothing.

https://www.fcc.gov/spoofing