Depend on the context, "同" as a preposition, can mean "with" or "for"
"我哋使唔使[同]佢準備個生日蛋糕" -- Do we need to prepare a birthday cake [for] him
"我哋使唔使[同]佢(一齊)準備個生日蛋糕" -- Do we need to prepare a birthday cake [with] him (together)
"同" strongly implies "for" in your example sentence. Birthday boy usually doesn't prepare the cake himself
"[同]我打電話叫警察" = "make a phone call to the police [for] me" (You don't need two people to make one phone call)
"同我去釣魚" is ambiguous, it can mean "go fishing with me " or "go fishing for me". To eliminate the ambiguity, you can say "同我一齊去釣魚 (go fishing with me together) or "幫我去釣魚" (go fishing for me).
Again with the context: If it is a friend saying to you: "同我去釣魚好唔好?", it would clearly be an invitation to you to go fishing [with] him; If you are a slave and your master say to you:"快啲同我去釣魚!" , then it would clearly be an order to you to go fishing [for] him.
As you can see, when "同" means "for" you can replace it with "幫" (help/ for) e.g. "[幫]佢準備個生日蛋糕", "[幫]我打電話叫警察",