Why not do something creative like have the investor sign a dollar bill (of any denomination) and give it to the founders? Every founder comes prepared with some cash in their wallet, and then when you confirm a deal the founders ask the investors to sign the dollar bill with a Sharpie/pen. On it would be some sort of short-hand for the deal valuation... Cash is more ubiquitous than phones - even impromptu, it's high…
Are you sure? Right now I have a phone but I don't have any cash.