Just like the Sprint HTC One, the Sprint HTC One Max can also be SIM unlocked for use on GSM networks. If you wish to use the One Max from Sprint on AT&T or T-Mobile, please note that it does not have compatible bands to get LTE on either of these networks.
LTE will not work on both AT&T and T-Mobile
HSPA+ will work on AT&T
Refarmed HSPA+ will work on T-Mobile
Here are few simple steps for unlocking the SIM. This method works only for HTC One Max from Sprint.
Disclaimer : Neither we nor the person who posted the method on XDA will be responsible for what you do on your device. Please read the instructions carefully before doing anything.
1) Unlock the boot-loader on your device through htcdev.com and flash any De odexed ROM like SkyDragon. If you are on Stock unrooted De-odexed ROM, Root your HTC One Max from Sprint using the steps listed in this post.
Finger print scanner does not work after unlocking boot-loader on HTC One Max. You can get it working by following the steps listed here.
2) Using any root explorer app and replace the HTCOMADM_SPCS.apk from /system/app on your device with this file.
3) Install SIM Unlock Helper from Google Play Store.
4) Open SIM Unlock Helper, Ignore “Device Check Fail” or “This Device is not a Sprint HTC One! Therefor, this Program will not run” warnings. Select “Display SIM Unlock Log” from Menu (by hitting 3-dots on upper right hand side), grant permissions when prompted. It shows “SIM Unlock Log”. Hit back button and now it says “Device Verified” and “Root Available” messages.
5) Tap start on SIM Unlock Helper to open OMADM Test Client, Scroll down through the menu and select the option “SIM LOCK”. Internally it sets the SIM Lock flag to unlock and it usually takes a couple of seconds. Your device will freeze during this time and returns to normal after the completion. Hit back button to view check SIM Unlock Result. If it says successful, reboot the device and it should be unlocked now and accept SIMs from GSM carriers.
6) Insert any SIM card (GSM) and set Network Mode (Settings ->Mobile Data) to GSM/UMTS only. Now your device should switch to GSM carrier. For mobile data services, You should create a new APN and set your GSM carrier specific values appropriately.
Here are the links for SIM unlocking HTC One and Samsung Galaxy S4 from Sprint: