I'm looking at the transcript on the companion site. The sequence goes:

1. Bussetta strangles Ola
2. Roth to be removed from hotel to hospital, with Busetta [edit] observing
3. Fredo goes to get a real drink
4. Bussetta stalks Roth at the hospital
5. Police march in and kill Bussetta

Timelines are flexible when the scenes switch back and forth, but I think the best bet here is that if Fredo tried to call someone at this point, then no one was there to answer. The best bet seems to be that Roth was going to be moved, someone went to tell Ola and found his dead body, so protection was ordered for Roth.