Why wont 3g work on my Straight Talk iPhone?

Text, calls, and wifi work fine but 3g wont work.

Added (1). I was told my phone runs off of Verizon towers.

You need a at&t version sim card from straight talk to get 3g support. Sounds like you have the t mobile version sim

Straight Talk uses T-Mobile's network. T-Mobile and AT&T use different frequencies for 3G data. You are limited to EDGE (2G) data only. MMS won't work well, if at all.