I’ve seen apps like TextFree which claim to give you a phone number for free, but I’m not sure how true that is or what limitations/catches there are. I assume they’d be invasive with the sign up process and their numbers are probably more likely to be blocked by online services for registration so I doubt it’s a good option. I believe Google Voice is free for personal use in the United States, but I think you’d already need to have a U.S. number to sign up, which costs money.
I’d encourage you to try to stick with more reputable VoIP providers and just find an option you can afford. JMP.chat is commonly recommended and it isn’t too expensive. There are cheaper VoIP providers as well but they tend to require personally identifying information.