This happens quite often if you use the OKX exchange strictly for depositing / withdrawing coins with small amounts, as an exchanger. The exchange does not like this and therefore may ask people to verify their phone number.
Probably what that's happened. I deposit on the exchange frequently and of low amount like $40 to $100. Probably what that caused it. Many people will prefer ti do it the way I did it too and a means it is very possible many people on OKX are doing KYC compulsorily.
I like OKX though, the exchange behave high liquidity, but about the KYC, I will not advice anyone to believe on those centralized exchanges like I said before, your coins are with them, they can make it impossible for withdrawal until you provide KYC.
I remembered when Binance made KYC mandatory, I asked people on rhis forum is OKX is better. I joined OKX, thinking KYC woukd not be requested for, but i was forced to do that. That is the point.
It might be interesting also to add the counties which are blocked from trading on these exchanges as most of them are not allowing US citizens to do so.
That Imis US, they prefer to just trade on exchanges where only US customers are present, except those p2p exchanges.