On any and every email client that I use my signature said: “Sent from a random place using my iPhone” Brevity is only allowed when your email comes from your mobile phone. People expect longer emails when they come from the computer or the iPad. Instead of writing longer emails, add that line for the signature (all your email programs, all your devices), people will understand the shorter email and you will be able to answer more in less time. One of the most important things is that all your devices said the same, nothing worse than people knowing from where you are sending emails.
i.e. My signature said: “Sent from a random place using my iPhone” but you can also said things like: “Sent from my Phone — please forgive the brevity and any strange auto-correct results I didn’t catch :)