I use skrill to sell btc. Before I give my potential customers my email to send money to, I ask them to give me their id document, e.g. passport, and screenshot of their profile page, so that I can see the names in id and profile match. I do this procedure to ensure the account wasn't hacked.

But this also makes customers who don't like giving this info for whatever reasons, refrain from me. I want to have another identification method, like sending sms to their number. If customer has access to his phone, he can enter the message I sent him, to me again.

Thing is I am not sure how skrill account can get hacked. I guess this can be done only via email: you get email, you ask skrill to reset your password and now you have access to other person's account. If you have skrill user's phone number, you can't do much with it... or can you? It looks like phone number is used only for verification when user creates skrill account. I don't see any way hacker can use it in case he knows what phone number is.