Could you tell me if it іs it correct and natural to say activate a cell phone plan meaning to get the minutes and internet that come with it? For example:
Sir, if you wish to activate your cell phone plan, what you need to do is just top up with 50 dollars.
If it doesn't sound natural, what would you say?
Activate would only be used for a brand-new plan. If the plan is frozen and needs the money get started again, then you would say reactivate. – Canadian Yankee
However, "top up with 50 dollars" sounds strange in American English. Top up normally takes an object, like top up the gas tank, or top up your account. We would probably just say add fifty dollars. There's a discussion of the phrasing here
That sounds natural to me. You could also say "start your cell phone plan".
