How to send a ttext message to Chinese cell phone

Hi, I’ve searched around for free text messaging on the web, stuff that doesn’t force you to sign-up.
I’ve found a few that allow you to send international text messages to cell phones for free but all of them I’ve found require you to provide the cellphone service provider. I don’t know this.
I have a very old friend in Hong Kong, they don’t speak very good English but I have their phone number. I know the no-brainer would be to call them, but the question is: How can I send an international text message (from the US) to a Hong Kong cell phone without knowing their service provider? Your help is appreciated, thanks

Why is it a big deal to call them and get the name of their service provider? You seem to be making this an unnecessarily complicated exercise.