Photo by Fabian Albert on Unsplash A few days ago they asked me for help to unlock an iPhone brought from the United States, but it had a SIM lock, that is, it could not be used by any cell phone operator in Ecuador. When inserting the Claro or Movistar chip, a message like the one seen below appeared: « Invalid SIM card . The SIM card that is installed in this iPhone is from a carrier that is not supported under the activation policy currently assigned by the activation server […].” Indicates that you have to ask the carrier that locked the iPhone to unlock it. The problem starts when you don't even know what that operator was. Without knowing this it is impossible to unlock the equipment..
Know for FREE which operator a cell phone belongs to: IMEI CHECK
There are many online services that help you find out in which company or company a cell phone is blocked. However, many of them come at a cost. For example, imei.info , one of the sites I personally trust the most, offers you this information in exchange for US $1.99 , although you have to put your credit card (there is no PayPal payment). There are also sellers on eBay that offer this information for $0.99 . I did not go to any of these options. Why pay for something you can get for free? Yes, imeicheck.com provides this information free of charge and immediately.
IMEI CHECK in this case offers free information such as warranty status (purchase date), SIM lock, Find my device status, blacklist, etc. 
If you do a check again, IMEI CHECK may give you new information. Simply access the site, " Check IMEI Free " option, enter the IMEI of your cell phone and tap on the Check IMEI button. You will receive an immediate and free report like the ones you see above (sometimes the report offers you more or less information, you can make several queries). It even gives you additional details like iCloud Activation Lock (Find My iPhone), if it's not blacklisted, etc. for free. If you look where it says Carrier/Sim Lock info , there is a link that says "HERE (FREE)". Clicking there will take you to the report we are interested in in this article and it will look something like the one below..
Here finally the Sim-Lock line indicates that the device is locked (Locked) and that the operator is AT&T United States. The information is correct, because I contacted AT&T by chat, where they confirmed this.
Pay to see the IMEI status of a cell phone?
Model: iPhone 7 128GB Black MM [A1778] [iPhone9,1]
Description: IPHONE 7 BLACK 128GB AT&T-USA
IMEI Number: 35XXXXXXXXXXXXXXX
Serial Number: C6XXXXXXXXXXX
Estimated Purchase Date: November 6, 2016
Warranty Status: Out Of
Age: 6 Device Warranty Years, 1 Month and 15 Days
Replaced Device: No
Locked Carrier: US AT&T Activation Policy
Country: United States
SIM-Lock: Locked
There is no major difference between imeicheck.com and paid services that offer practically the same information (in fact even less than the free service). Above you can see the report from unlockboot.com . They charge $2.99 for the IMEI (carrier and lock status) check..
MEI Number: 35XXXXXXXXXXXX
Model: iPhone 7 128GB Black
AT&T Status: Not Found
They also check the status of the account with the operator, which is seen above, for 4 dollars . As you can see, they did not find any information, but they still charge for the check. The advantage of unlockboot.com is that they accept PayPal payments.
And how should I unlock my cell phone?
Obviously this is another topic, but in summary it is best to go to the operator to request the unlocking. In my case I contacted AT&T. They offered me support in Spanish and told me that they can unlock the device for free, as long as the number associated with the device is free of debt. They need the AT&T account phone number and password for this, or if this is not possible, they need to contact the owner of the number the device was associated with. This will obviously vary from operator to operator. There are also other apparently "clandestine" methods or services that are offered online to unlock cell phones blocked by operators, with prices that vary greatly from one to another, which apparently can be done remotely, something known as RSIM, etc.,
What it means and how to avoid network lock (invalid SIM) on Samsung
This APP disables the SIM network unlock PIN on the Samsung Galaxy Note 3 (root)
How to use both SIM and micro SD at the same time in a modern Dual SIM phone
