If you turn on your phone in another country AND your data is turned on, then as soon as any data passes from your phone to the network your data roaming.
If you turn off data and your phone is on and you get a call you will get voice roaming charges.
Also, depending on your plan, some carriers require you to contact them in advance of a trip to enable international roaming. So not sure if your going internationally or domestically.
If you dont turn on international roaming theres a chance your phone wont work as your carrier has not authroized your account to roam internationally.
Ive heard of people who often buy a prepaid sim / sometimes phone just to keep in touch with data services and sometimes phone/text. Its usually cheaper then paying a major carrier international roaming.