it's a magical world

I suppose that could be true. I assumed it was the Ring calling him, but yes, that's usually inspired by the Nazgul. Now that I think about it, yes, I DO think the Nazgul want him to claim the Ring. They (and Sauron) can always see him more clearly when he's wearing the Ring, and that's just when he casually slips it on. Think RotK. The INSTANT Frodo claims the Ring, all hell breaks loose, and I'm not talking about Gollum. Sauron goes beserk, the Ringwraiths abandon battle and go tearing off towards Mount Doom. If Movie!Frodo had claimed the Ring in Osgiliath, he would've been toast within seconds.