There really is no solution except to go through Customer Support. Phone verification is usually pretty basic, the one thing you have to be very cautious of is too many failed attempts close together. If you have a normal failure that you think might be due to mis-typing the number or something basic, it’s good to wait several hours before trying again so the system doesn’t detect repeated failed attempts that look like hacking.
That typically resolves any minor issues. If that doesn’t work, it’s usually a bigger issue like trying to verify a phone number from one country with your IP in another, or using VPN on your computer but verifying with your normal home number, etc. I’ve heard of one case where someone tried to use a “burner” mobile phone and that failed but they re-tried with the landline method and it worked fine. So, basically, the more simplistic your setup, the more likely you succeed. Good luck.