PHONE is an APN that AT&T umbrella'd all types of data connectivity under. PHONE will automatically choose to connect to the fastest available network in your area. PTA is what they were putting on phones when LTE phones were first being released, but since then, PHONE is the primary APN one should be using. Using PTA won't hurt anything, and should work fine, but why bother if PHONE is already programmed in there.
At least, that's what they the us at my AT&T Data Support call center I work at :/
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