How to text friends in USA?

Context: I live in Hong Kong but attended a summer camp in the US.

After returning home I have attempted to text my friends that I have made at camp, but none of them have yet replied. A quick google found that the messages may have been sent as iMessages, since I used to own an iPhone, so I disabled iMessage for my phone number. I'm using ad Android phone and they all have iPhones. I have added a +1 in front of all their phone numbers, but I'm not sure if I'm doing something wrong, or if the messages are even being delivered to them.

Can you use WhatsApp?

English is the primary language of the United States

Skype. Best and free too. And no need to register a phone number.

With a phone