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Call forwarding error on AT&T with Android phone
Sometime in the past 4 months, my Android phone stopped being able to configure conditional call forwarding (busy/no-answer/unreachable) using either the standard **004 USSD/MMI feature code or using the Phone app's settings menu (Phone > Settings > Calls > Call forwarding > Voice).
When I try using the **004 feature code, I get the following error every time: "Call forwarding: Connection problem or invalid MMI code."
When I try using the Phone app's settings menu, I get the following error: "Call settings error: Network or SIM card error."
Oddly, when I put my AT&T SIM card into a different model Android phone, both methods work without a problem. They just won't work on my usual phone, a Blackberry Key2 (Android). The **004 feature code worked on my same Key2 as recently as last September, when I first joined AT&T, but it stopped working sometime since then.
I exchanged my AT&T SIM card for a new one, but it made no difference. I've tried rebooting my phone in Safe Mode, disabling various apps, and even factory resetting it, again with no difference.
I have triple-checked that I am using the proper USSD/MMI feature code sequence, **004*1xxxyyyzzzz#[SEND], where xxxyyyzzzz is the target phone number). I have already tried using different target phone numbers; that is not the problem. And, I have tried using various other USSD/MMI feature codes (**21, **67, *004, #004, **002, etc.), and none of them seem to work on the Key2.
Any idea what's going on? Having to call AT&T customer service every time I want to change my conditional call forwarding number or other phone settings is not a good solution.