iPhone Invalid or No SIM

Introduction

In order for your iPhone to receive a signal from your cell phone provider, it needs a SIM card to identify itself to your carrier's network. If your phone cannot detect, or use your SIM card, you're not going to get very far if you're out of WiFi range. Fixing this issue is usually simple, but in the case that it's not, we've covered all the bases here.

If you have an iPhone 7 model A1660, A1780 or A1779, Apple has a program that may address your issue. Although the program specifies No Service, it can also encompass SIM not detected and Apple may repair this for you at no charge.

First Steps

Before undertaking any of the more involved solutions below, here are a few quick fixes to get you started.

Restart your phone. This can clear any minor software hiccups.
Update your OS. You can check for new versions under Settings > General >Software Update.
Check for a carrier settings update. Occasionally, your cell phone provider will push updates to the way your phone can connect to its network. To prompt a cellular update, be sure you're on WiFi and navigate to Settings > General > About. If an update is available, it will initiate a pop-up to install it.

Causes

SIM Card

Your SIM card is actually a small computer chip embedded into plastic so it can be transferred easily across devices. Although they are resilient, they can become faulty, or may not be compatible with the phone you’ve transferred it into.

Try a simple reseat of the card. Eject the SIM tray. Make sure the card is properly oriented in the cutout. Reinsert the tray and retest.
Try the SIM from a friend or family member with the same cellular carrier. If it works, replace your SIM card. You can request one from your service provider, or visit one of their physical locations.
As a workaround, use your phone’s eSIM instead. Starting with the iPhone XS, all iPhones can handle two SIM cards—one physical card, and a second which is usually embedded in the phone's logic board. This electronic SIM may work when a physical SIM will not. Verify that your carrier supports eSIMs and contact them to activate it.

Phone is Carrier Locked

Especially in the US, where the cost of a cell phone is often paid off in installments, service providers implement locks to prevent you from utilizing your equipment with competitors. At least until they have recouped the cost of the phone.

Note: This will only occur when inserting a SIM into a new device or a new SIM into your current one. If your existing SIM suddenly stopped working, skip this section.
When buying a used phone, check with your carrier to ensure it can be activated on their network. You will need the IMEI for the device. This will validate compatibility and can help avoid surprises.
If it is compatible, but currently locked, contact the original carrier for the phone to see if it is eligible for unlock.
Be cautious of online services claiming to unlock iPhones. These fixes may work initially (if at all), but are easily undone via update or even restarting your phone.

Software Bug

Occasionally, incompatible or corrupted software can cause network issues with an iPhone. It is infrequent, but worth ruling out before embarking on deeper level hardware fixes.

Perform a Network Settings reset. Be aware this will clear all known WiFi networks and Bluetooth devices. You will need to reconnect them afterward.
Update the operating system to the most recent version.
To rule out software altogether, reset the device. In cases of a potential operating system fault, do this via a PC with iTunes or a Mac to be sure a fresh copy of iOS is installed.
- Note: Proceed with caution. This may break the ability to activate your phone if the issue is hardware related. Activation is necessary to complete setup, so If this happens you'll be unable to get to the home screen. Be advised this is a possibility.

Faulty SIM slot

This occurs most frequently on newer models of iPhone. Beginning with the iPhone XR, some models feature a Apple iPhone Parts. On previous models, the slot is soldered onto the logic board.

The SIM slot is a frequent entry point for liquids, despite the gasket Apple has utilized since the iPhone 7. Verify the liquid damage indicator inside the SIM slot has not been triggered.
In cases of SIM slot liquid entry, replacement of the SIM slot is advisable if it is a modular component on your iPhone.

iPhone 12 modular SIM slot

iPhone 6S soldered on SIM slot

Logic Board Fault

Logic board faults related to cellular function are not uncommon, especially after a hard drop. The majority of iPhones X and newer make use of a dual logic board system. The two boards are stacked atop one another and connected via an intermediary board. Nearly all cellular functions are on the bottom board, and a hard drop can sever communications between the two boards, resulting in a multitude of issues.

Check for signs of bend, burn, or corrosion on your board, which may be the underlying cause.
If no other fixes have made a difference, the board is likely to be the issue. Replacement is the easiest solution in this case.
It may be repairable on the board level, but microsoldering skills are necessary. Seek a local repair shop with these skills if this is outside your skill set.