I remember you saying even the H20 Sim needs to have an 'unlocked' phone for it to work? Or was that only for the data to work?
Data APNs are typically locked down in software on most all handsets (including AT&T) when the handset's hardware SIM card reader is also carrier locked. This way, even if you switch to an AT&T MVNO with an AT&T postpaid locked handset, it prevents the user from gaining mobile data access from a cheaper provider on their own network without paying off the phone, completing your contract, and kissing Ma Vader's rings before going on your merry way. This is also how they keep you from buying a cheaper subsidized handset from say AT&T's prepaid GoPhone and Cricket divisions and taking it to another, cheaper AT&T MVNO if you want/need data access or MMS support without first keeping it active on a GoPhone/Cricket account for 12 months to recoup the subsidy before allowing you the freedom to leave through carrier unlocking. It works this way on the T-Mobile end as well as other GSM providers around the world who practice carrier locking, too.
The thing is, most MVNO SIM cards are typically recognized by the handsets as SIM cards from their parent network on the hardware level. Airvoice and H2O SIM cards are recognized as AT&T, P'tel and Ting as T-Mobile, etc. Because of this, it'll let you make and receive calls and SMS messages even with a carrier locked handset using an active SIM card from an MVNO on the same network as the phone is locked to, but APNs for mobile data and MMS cannot be changed on the software end due to the carrier locking. Due to this nature of the GSM hardware, they may not be able to prevent the phone calls and SMS messages on these locked handsets with other MVNO's SIM cards on their network from working short of blacklisting the phone's IMEI (which they will do), but they
can still cripple it by completely blocking data access and configuration.
(Fun Fact: You can actually use an unused AT&T SIM card to activate service with most AT&T MVNOs such as Airvoice Wireless.)This is where it's gotten messy for iPhones. With T-Mobile network SIM cards, if the handset is carrier unlocked, the APN settings are accessible for manual configuration and there's no data configuration trauma with
any T-Mobile or T-Mo MVNO. With AT&T network SIM cards even in an unlocked handset, however, the APN settings remain hidden away and inaccessible... and it's been this way and only gotten worse since iOS7. Clearly, this creates a problem. Unless your AT&T MVNO is on Apple's officially blessed provider list (Consumer Cellular, Cricket, GoPhone, StraightTalk) which auto-configures the APNs for you based on the carrier detected in software, any other AT&T MVNO SIM will effectively set these phones to AT&T postpaid APNs by default upon insertion due to their lack of official auto-configuration support from Apple.
Unfortunately, because you
also have no way of manually configuring the APNs for your AT&T MVNO (Airvoice, H2O Wireless, Puretalk USA, Red Pocket, etc.) due to this setting lock-out "auto-configure" feature, you
still have data configuration issues even after carrier unlocking. This is where needing to use unlockit.co.nz just to set your data APN now comes into play. To make matters worse, the MMS settings for these non-blessed AT&T MVNOs typically can't be configured most of the time now as well even using unlockit without jailbreaking. It's a hot mess, and it's why I typically don't recommend iPhone users who need MMS support to use
any AT&T MVNOs from the guide anymore except for Consumer Cellular.
Make sense?
