Translation:Take the phone from him and run back.
Perhaps someone needs his phone and somebody else came over and asks you to "Take his phone and run back" (or "Take the phone from him and run back"). I come from a large family and we do these kind of things often when we are together (for example, taking the keys and running back to go get something while the other person does something else). Also, if you've ever had to organize a large event with a ton of people, this situation is possible.
I'd say there's nothing in the Russian sentence that implicitly says the phone is his, so it could be mine too. A scenario without stealing: Someone picked my lost phone and I have to ask a friend to get it for me because I'm already buying my ticket at the train station or I have to finish packing my things at the hotel.
DL is out of levels to reply with.
I can easily imagine that my wife and I just met someone interesting, talked a bit, then left. Then we remember, we forgot to get his/her phone number! So, "Go back and get his phone number!"
Not weird at all, in English at least.
So, back to my same question: Would you assume this is: an instruction 1) to "take his phone away from him" or 2) to "get his phone number"? Is it unclear? Maybe it can work either way if nothing else is said?
I said what I would assume. I guess, both are possible. The whole thing depends on cooperation of both parties: the person should be willing to give the phone (for example, because it is your phone that you left) and/or the phone number (people are under no obligation to share their phone number with anyone that asks).
Of course, for asking the phone number you can also use "Спроси у него телефон/Спроси его телефон" or "Узнай у него телефон", and for a physical phone you can use "Забери", which is also unambiguous regardless of the century.