Гуднуть: to honk, sort of
When we talk about the "untranslatable" in texts, we mean words and concepts that do not have an equivalent in another language: cultural concepts, names of food or types of architecture, etc. But it turns out the untranslatable might also be associated with something as mundane as cell phone payment plans.
In the U.S., you get a bill once a month for your phone activity over the past 30 days. In Russia, you make a pre-payment for phone traffic you expect to have over the upcoming 30 days. If you forget to pay or spend more time yakking than usual, you may suddenly realize you are really low on phone funds or have hit zero.
The ways Russians deal with these two circumstances are described by a variety of slang words — all of which are "untranslatable" into American English because in the U.S. there is not the annoying problem, the clever solution or the cool way to talk about it.
In Russia, if you are low on funds but need to talk to someone, you call them from your phone, let the call ring once so that your number flashes on their screen and then you hang up. This will let the other person know to call you back. My young friends in Moscow call this гуднуть (literally, to honk once) or сделать дозвон (literally, to make a call through). Or they say: Я позвоню и сброшу звонок, и ты мне сразу перезвонишь (I will call you and drop the call, and you call me right back). Or they might explain: Я тебе просигналю, и ты мне позвони (I will signal you, and you call me back).
In other parts of the former Soviet Union — and possibly among some folks in Moscow — this is called маякнуть from the noun маяк (lighthouse) and the notion of sending a signal. Although I cannot independently confirm this, I have read that Russian speakers in Latvia use the word хрюкнуть (literally, to oink once). I hope that is true because it is very witty.
If your cell phone account is on zero or in the red, your provider lets you send a couple of emergency text messages free of charge. My young Moscow friends call this кинуть халяву (literally, to toss a freebie), but in other regions and other sets it might be called кинуть бомж-смс, попрошайку or бесплатку (literally, to toss a homeless text message, a beggar text or a free text).
You can send out slangy signals in other Russian contexts. In Ukraine маякнуть is commonly used to mean to send a signal, inform or report something: Маякни мне, когда выйдешь (Let me know when you are leaving). No one in my Moscow set says this. Instead, they use the word вякнуть, which is "to bark" in literary Russian and "to talk nonsense" in older colloquial Russian. In today's slang, it means "to give a holler": Вякни, когда доедешь, а то я буду беспокоиться (Let me know when you get home, or else I will worry).
Another slang word for this is семафорить (to signal, as in workers or sailors with flags). Не забудь просемафорить мне, что всё в порядке (Do not forget to give me a sign that everything's all right).
Маякнуть, which is a busy little word, can also mean to flash your car lights: Маякни ему дальними фарами. (Flash your high beams at him.)
Flashing, barking, honking, oinking … Signaling is energetic work in Russia.