I think Fredo's "Wrong number" explanation of his phone call from Ola is supposed to show everyone that Fredo really is that stupid.
[Edit} I (obviously) don't know what Ola told him would happen, but one possibility is that they wanted the drapes open so they would see when Michael entered the bedroom so that they could search for papers or some such in Michael's office.

I reckon most people including me under the circumstances would have got flustered! The hit on Michael is botched, then Ola who has got Fredo in 'deep enough already' phoning home late in the night “We need some more help” Perhaps veiled threats “Listen good Fredo” “Your brother's not going to find out we talked” etc.