How to add a line to your phone plan: steps, real cost and the catches

Adding a line is how most families get a child, a partner or a parent onto a plan. It is usually quick, but the final cost has more parts than the advertised line price: taxes and fees, a new phone payment, an activation fee and sometimes a changed price for every other line. This guide shows the steps that work on any major US carrier, what to check before you say yes and how to avoid the catches that raise the bill later.

Before you add a line: check these five things

How to add a line in the carrier app or website

Bring your own phone or buy a new one?

Moving an existing number onto the new line

If the person already has a number with another carrier, you can move it to your plan. They need the old account number and a transfer PIN from the old carrier. See how to get a number transfer PIN. Enter both when you add the line, and keep the old service active until the move is complete. A number can move in minutes or a few hours.

Adding a line for a child

Many carriers offer a lower-cost plan or a watch line for kids, and some have parental controls included. Decide whether you want a full smartphone, a basic phone or a smartwatch. Check which parental controls the carrier gives, and which you can set on the phone itself. For an iPhone, see how to set up an iPhone for a child. For an Android phone, you can use Google Family Link, and you can lock the screen to one app for young children with screen pinning.

What if you are not the account owner?

• Ask the owner to add the line, or to make you a manager so you can do it.
• Some carriers let a manager add lines but not change the plan.
• If the owner is not around, you may need to open your own account instead.
• Carriers may ask for the owner's PIN or a one-time code to confirm.

Add a line or open a separate account?

Adding a line to an existing plan is usually cheaper per line, but it ties the person to your account. If someone moves out, wants privacy or may switch carriers, a separate account is cleaner. Think about who owns the phone payments, who pays the bill, who can make changes and what happens if you stop paying. For adults who are not family, a separate account avoids problems. For children and partners, a shared plan is simple and cheap.

Common problems when adding a line

• Phone not compatible: check the IMEI with the carrier, and if the phone is locked to another carrier, ask them to unlock it.
• Number transfer fails: the PIN may be expired or the account number wrong. Make a new PIN and try again.
• eSIM will not activate: connect to Wi-Fi, restart the phone and try the carrier app. See the eSIM guides for step-by-step help.
• Credit check required: a new payment plan often needs one. Bringing your own phone may avoid it.
• Bill is higher than expected: check taxes, fees and any per-line price change, and ask the carrier to explain each charge.

If the bill creeps up after adding the line, read how to lower your phone bill for ways to trim it. If you are not sure the plan fits your family, choose a cell phone plan with a checklist before you commit.