I’ve had a similar issue with registration forms before. I think using someone else’s number is not worth the trouble, especially if the casino later asks for SMS verification. I would contact support first and ask if they accept a landline or another verification method. If they don’t, I’d simply choose another casino.
I think RTP is useful, but it doesn't tell you what your own results will look like. I came across this topic on Gamblizard recently, and they explain the difference between RTP and variance in more detail. RTP shows the long-term math, while a normal session can go completely differently.