I've been on ST for about 2 years, using 99% iPhones, which were very hard to set up (back when ST started). Now, I have S5 Active and iPhone 5 and both work very well.
You don't need no $70 BS activation package. Just go to http://www.straighttalkbyop.com
click Option 2 and buy yourself an AT$T compatible SIM card, assuming you have an AT$T S5. The SIM card is LTE ready, costs $6.99 and ships FREE via FedEx 2-day service.
1. If you're getting a new phone number, register your sim at www.straighttalk.com
when you receive it and system will assign you a new number.
2. If you are porting your old phone number you can still do it from straighttalk.com (you'll need your old carrier account password or access PIN for that). Avoid calling ST, they have horrible CS. It will take an hour to set it up and this is if you understand what CS rep says 100% of the time which is close to impossible
Their website is easy to navigate, you'll figure it out.
Now, pop the SIM in and enjoy your new $45 *unlimited* service. You will need to create a new APN entry for ST. Here
you can get the right APN settings.