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I've seen mixed things also looking it up for straight talk. But if you are using ATT Straight talk sim (As opposed to straight talk iPhone for Verizon) it should work. If it works be sure to report back, sure a lot of people would like to get a confirmation if considering switching.
I installed the Youmail Visual Voicemail app over the weekend, but only just activated it. The activation was easy and quick, the app adds a contact to your address book (with your permission) that you call to start forwarding voicemails to Youmail. Deactivation is the same process, I haven't tested it but the app creates a contact to deactivate voicemail forwarding.
Once you activate, all your voicemails are routed into the Youmail system. You will need to create a new greeting for voicemails (the default one is fine though, it even pronounced my name right.)
The visual voicemail seems pretty straightforward, Youmail offers to make a test call for you, and I also made a test call from a landline. Both calls showed up in my 'Messages' list, and the app gives you notifications with the caller info. (there are 5 tabs at the bottom of the app- Messages, Hang Ups, Greetings, People, and More (activation/deactivation, notifications, etc.) And hey- I just noticed you can save different greetings, and even create custom greetings for specific contacts, etc. I think I will record a funny one for my wife...
Since voicemails are being routed to the Youmail server/service, your existing Straight Talk voicemail box will be empty. So the one disadvantage is you are using a separate app to check your voicemail (not a big deal IMO since the whole process seems pretty smooth.)
If any hiccups or problems come up I will post here, but otherwise the app seems like a pretty painless (and free) way of getting visual voicemail back for Straight Talk!
So as of now, I have a pretty awesome setup with Straight Talk on my ATT iPhone 5 running iOS 7.02 with a newer LTE-enabled SIM card-
1. No messing with settings, SIM swaps, or even installing an APN.
2. Data (LTE) works
3. Presumably MMS works (most of my contacts use iMessage anyway, so I haven't been able to confirm.)
4. Visual Voicemail works with Youmail Visual Voicemail app and service
5. Absolutely unbeatable price for moderate data usage (I buy reloads on EBay to bring cost down to $30-35 a month.)
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Go to Settings>General>Profile and Remove Profile
This removes a user-installed APN profile, which is totally different from carrier settings updates.