I think rav moshe feinstein allowed answering amen or havdala over a phone. Would this apply to cell phones which dont create sound thru vibration?
I would assume that it does. Cellphone, like telephones accept the electic current and transmit the signals that they recieve into sound, which is caused by vibration. Regardless, the idea is that you know clearly that the person said what he did, and that is why he held that b’dieved you can be yotza havdala. therefore a cell phone would not be differant that a regular one regarding this.
Igros Moshe O:CH 2-108, 4- 91(4).