How to follow up phone interview?

Don't.   Thank yous for interviews went out of style 20 years ago.

The person who interviewed you was just doing their job.   They received a paycheck for it.   They were not doing you a favor.

The ONLY reason you would be sending a thank you is because you think it will help you get the job.   It's 100% self-serving.   The person who interviewed you knows this.   Self-serving thank yous are pointless interruptions and may be interpreted as insincere or even rude.

Seriously.   Why would anyone want to receive a thank you from every applicant they interview?    Spend 20 years hiring people and you'll see how silly it is.   NO ONE says, "Hey, that one sent a thank you note so maybe I should hire him/her."    It just doesn't happen.

You could get clever.  look at the address format of the HR person (is it FirstinitialLastname@company.com, firstname.lastname@company.com, etc.) and see if you can guess the manager's email address. 

Or, you could mail a physical thank you card to the office addressed to him/her. 

Or you could ask the HR person to thank the manager on your behalf. 

Or, you would wait and see if you get an in-person interview before worrying about sending a follow-up thank you.