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Reading this thread, @evomaster seems to be the only person who replied who knew specifically what the OP's friend may have been talking about: LTE SIM compatibility between Verizon and AT&T.
Originally Posted by evomaster
The fact is we don't know if LTE Sim's will be cross carrier compatible. With them both deploying LTE in the 700Mhz frequency,there is a high probability that a device, if it can be radio "unlocked" purchased for one network may work on the 4G of the other. However, without VoLTE being deployed, this would be for data services only, as AT&T and Verizon's 3G and voice services are completely incompatible.
However, if VoLTE (Voice over LTE) comes into play, this may be a viable option in the future. We'll have to see if VoLTE will be a software/firmware upgrade, or require chipset changes for the big two.