Suspicious activity

Apple does not make such calls.

All such calls are scams from criminals attempting to steal your personal and financial information. The callers will often use number spoofing to pretend they are calling from a legitimate business. The correct course of action is to hang up on them, repeatedly if needed. 

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Memekristie Said:

“Suspicious activity: I have had two phone calls indicating that there has been suspicious activity on my Apple ID like someone is trying to hack into my account.”

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What to Do:

These phone calls you speak of are no more than a scam. So, Report them to Apple...

Reporting this to Apple:

Take a screenshot of that phone number and send it off to Apple.

Go Here: Take a Screenshot on your iPhone - Apple Support

Forward the screenshot to: reportphishing@apple.com

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Learn More Here:

• Detect and Block Spam Phone Calls - Apple Support
• Recognize and Avoid Phishing Message, Phony Support Calls, and Other Scams Apple Support
• (In the US. Only): Report Phishing Scam Calls at www.ftccomplaintassistant.gov

And, follow what stedman1 said up with What to do after you change your Apple ID or Password https://support.apple.com/en-us/HT204071  (supplementary to what the fist link says)

You're quite welcome.