Set up iMessage and SMS/MMS on iPhone
In the Messages app , you can exchange text messages using SMS and MMS through your cellular service on iPhone, and with other iOS devices and Mac computers using iMessage.
iMessage is an Apple service that sends messages over Wi-Fi or cellular connections to other iOS devices (with iOS 5 or later) and Mac computers (with OS X 10.8 or later). These messages don’t count against your messaging plan. Messages sent via iMessage can include photos, videos, and other info. You can see when other people are typing, and send read receipts to let them know when you’ve read their messages. For security, messages sent via iMessage are encrypted before they’re sent.
iMessage texts appear in blue bubbles, and SMS/MMS texts appear in green bubbles. See the Apple Support article Use Messages on your iPhone, iPad, or iPod touch.
Sign in to iMessage
Go to Settings > Messages.
Turn on iMessage.
On models with Dual SIM, tap iMessage & FaceTime Line, then choose a line.
You can register only one of your phone numbers for iMessage and FaceTime. Your other number can’t send or receive messages using iMessage, or make or receive FaceTime calls. See also Manage your cellular plans.
Sign in to iMessage on your Mac and other Apple devices using the same Apple ID
If you sign in to iMessage with the same Apple ID on all your devices, all the messages that you send and receive on iPhone also appear on your other Apple devices. Send a message from whichever device is closest to you, or use Handoff to start a conversation on one device and continue it on another.
On each iOS device, go to Settings > Messages, then turn on iMessage.
On your Mac, open Messages, then do one of the following:
If you’re signing in for the first time, enter your Apple ID and password, then click Sign In.
If you signed in before and want to use a different Apple ID, choose Messages > Preferences, click Accounts, then select iMessage in the Accounts list.
With Continuity, all the SMS/MMS messages you send and receive on iPhone also appear on your other iOS devices and your Mac. See the Apple Support article Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch.
Use Messages in iCloud
Go to Settings > [your name] > iCloud, then turn on Messages (if it’s not already turned on).
Every message you send and receive on your iPhone is saved in iCloud. And, when you sign in to iCloud on a new device that also has Messages in iCloud turned on, all your conversations show up there automatically. Because your messages and any attachments are stored in iCloud, you may have more free space on your iPhone when you need it. Message bubbles, whole conversations, and attachments you delete from iPhone are also deleted from your other Apple devices (iOS 11.4 or later and macOS High Sierra 10.13.5 or later) where Messages in iCloud is turned on. See the Apple Support article Keep all your messages in iCloud.
Note: Messages in iCloud uses iCloud storage. See Manage Apple ID and iCloud settings on iPhone for information about iCloud storage.