mobile

I'm using straight talk on an at&t s6 edge. Trying to get the google voice voicemail to work. I've tried the mmi code **004*gmnumber# to activate conditional call forwarding, but I always get "couldn't get server response".. Trying to change the voicemail service provider also gives errors. "Can not read data. Network or sim error"

I have tried the individual mmi codes for different kinds of conditional call forwarding, but get same error:"couldn't get server response".

I have read that at&t controls the conditional call forwarding service, being the original carrier for straight talk, but because I don't have an active account, they can not change features for my number.

The ST rep tells me that there might be a hardware specific mmi code that Samsung may provide?

Does anyone have any solutions?

Thanks guys,
Gozirra