I "Dis donc" is used to attract attention, or rather as a signal to make someone nearby aware that they are being solicited, that someone is going to talk to them. This is the plain use of this interjective locution. However, it seems that people do not use it in this way if they do not know one another (unless "dites donc" has another function, see III).
- Dis donc, quand tu auras fini ton livre tu peux me conduire au supermarché ?
It seems that "hey" shouldn't be used to translate "dis donc" in this context, but I am not sure of this. A good English equivalent could be "say".
II Another use of it is totally different; the two persons involved have been conversing for a while and there is no need to attract the attention of anyone. Given this context, "dis donc" is used to declare to the other person something that departs from the normal flow of conversation.
— Il n'y aura pas eu beaucoup de neige cet hiver…
— Je ne m'en plains pas, j'aime la chaleur et le soleil.
— Un temps trop chaud n'est pas agréable non plus, je ne le supporte pas.
— Dites donc, vous savez que la prime de vacances d'été va être supprimée ?
"Say" could again be used or "by the way" among others that I don't know of. I don't know about "hey", though it is an interjection used to draw attention.
III A third possibility of use of this locution is to begin a reply in a verbal exchange where the person being talked to has just gone beyond a certain limit in the way of not respecting either a standard of politeness or in the way of putting forth a point of view that is unacceptable, demeaning, based on lies, viciousness and so on. When "dis donc" is used thus, it is more often than not pronounced with more or less force, in a manner that never fails to be curt. A second context is that of the abnormal behaviour of someone that you know or just as well that you don't know at all, but that has just behaved in very uncivil manner (talked rudely to a little boy, threaded on your feet without apologising, dropped intently trash on the floor, etc). In that case people might use "dites donc" in the same way as explained above but possibly in a more angry manner, that being meant both to make it clear to the person misbehaving that someone is adressing them and to reproach them strongly with doing what they just did.
— Dites donc, vous ne pouvez pas vous excuser lorsque vous bousculez les gens ?
Dites donc ! Si vous ne pouvez pas finir votre sandwich c'est dans la poubelle qu'il faut le jeter, pas sur le plancher.
It seems that "hey" will not do, except in the second example if "you" is added (Hey you !)
IV The dictionary says that in the context considered "justement" is meant to stress a coincidence, but personally I tend to think it should be a coincidence of facts inherent to the subject being discussed (people apparently do not take necessarily that into consideration).
(TLFi) 2. [Marque une coïncidence, une corrélation] Précisément. Synon. juste
— Il semble nécessaire de renouveler cet abonnement, il n'est que de douze mois.
_ Justement, je viens de recevoir un avis du magazine, trente jours de délai.
A translation could be "precisely", "exactly" or even "that's it".
In those cases when the coïncidence is not intrinsic to the subject discussed, I think another choice of locution is more expressive while at the same time it keeps the two ideas well separated.
— Il y a une fête foraine dans ce quartier, j'ai traversé la place où elle a lieu … je crois que je n'irais plus jamais passer du temps dans un de ces endroits, comme lorsque j'étais jeune.
— En parlant de ça, Léone m'a dit qu'elle était minable cette année et qu'elle n'a pas l'intention d'y mettre les pieds.
Instead of "en parlant de ça" could be used "maintenant que tu parle de ça" but it might be colloquial. A little longer but certainly not colloquial would be "maintenant que tu parle de ça, je me souviens que".
In English, I think that "now that you mention that" would be perfect for a translation.