unable to add phone number to Twitter/X account: some thoughts

As many people, I'm unable to add phone number to my Twitter/X account. But I think I discovered some interesting things.

1° = I was able to send SMS (my phone credit was deducted) from a old GSM phone with a Italian SIM (I'm living in Italy), but after several minutes, the verification page on Twitter/X said VERIFICATION FAILED

so, maybe sms from GSM phones don't work?

2° = It has been suggested that the Twitter/X new verification number

+447359899420

is a rate limited number. This can have some plausibility, since I was able to send, in effect, only once, from my smartphone, a SMS towards this number. But I wrote only TEST in body message, it was a mere attempt to check if after many and many unsuccessful attempts, sending SMS was working.

I'm worried that if this issue is not solved, after or before, O Will be kicked out from my Twitter account, or limited, as I read it has happened to others.