Can't Call AT&T phone from Verizon network, etc, Vice Versa (Straight Talk, Trac Phone, etc)

This was a confusing issue I was having.

I could not call some mobiles but some worked fine and land lines were also fine.

The quick answer is this on the Android Phone: Settings > Mobile Networks > Network Mode > Select anything but Global. (You should probably select appropriately for your network, ATT GSM, Verizon CDMA)

I must have talked to like 10 straight talk Reps and none led me to do this, I was at the end of my rope and going to just change providers.

I came across a post that hinted at this being an issue between Samsung and Iphones and turning off LTE would fix it, but you do not have to do that just make sure your phone is not in Global Calling mode.

Regards and I hope it helps.