Hey, badankles. The steps provided above may help when the unlock signal was sent and the phone is still showing "Invalid SIM card error". If you have the same issue, performed the steps and problem persists, please feel free to let us know to investigate what could be causing it.
@Genci, thanks. I've done that before so thought there were something else. Glad it worked for you.
@Sprint_Palmira, I tried the same steps. (Note that a few months ago, I've contacted Sprint via phone call and the agent said they would unlock, no problem, but it never showed unlocked after initiating the UICC Unlock option. I just haven't made time to call back.) The first two were successful (after a phone restart), but again, the last step UICC Unlock came back with the same message, "You can currently only use your service provider UICC with device. To use a different UICC, contact your service provider."
Thanks for letting us know. Have you tried doing these steps with the Sprint SIM card in the Samsung device? Please try and let me know for further assistance.
@Sprint_Yanil, yes, that's with the Sprint SIM card because the phone is still on Sprint. I did the steps with both WiFi off while connected to the Sprint network, and WiFi on. Both resulted the same with the last step.
For devices that are not SIM Unlock-capable, such as your Note 4 and others introduced before February 2015, Sprint can only provide a Master Subsidy Lock code to eligible customers.
Please note, while Sprint can provide the MSL code, this does not mean that device is unlocked or that it will be accepted by another carrier or interoperate with another carrier's network. Whether another carrier will accept and activate a device is subject to that carrier's policies and capabilities.
The UICC is still locked to sprint. I had the same issue with a S7, once the unlocked the UICC it was able to work on other networks.
UICC Unlock: Unlock your phone’s Universal Integrated Circuit Card
I know this is possible as Samsung Galaxy S4 are able to UICC unlock and that phone is even older than Note 4