Unable to switch back to using the handset on a Speakerphone?

I've got a Zenex ZN-TP5143 corded Speakerphone and I am no longer able to use the handset to talk. I am able to call and answer the phone only if I use the speakerphone button.
I wrote to the company asking if there is a way to re-set the phone but they didn't respond to the question.
I unplugged everything and re-attached it but it doesn't work.
I doubt that it is the handset - I think that I probably pushed a wrong button but I don't know what I did.
Thanks!