Keep in mind most of those “free sms” apps or those text services do not allow text verification messages. They are generally only for actual texting back and forth. I know because I had to make an Apple account and found out you cannot do it without using an actual cell phone number.
Your best bet is to just buy some prepaid SIM card and like $10 of airtime and use that for verification. Because I am pretty sure none of those free services will work for what you are actually trying to do.
That's probably the best solution and I think that these free sms things have a limited users that can use the number right? So if I was OP, I wouldn't waste time finding one because when free is being asked online, there's a large chance that scammers will try and take advantage of your naiveté on the service. My advice to OP is to follow what @adaseb is saying, much safer and it's not that expensive of a deal for you, just have an ejector pin on the ready to swap out sim cards.