General Query

You must call for your account number. It is not available online. Not sure why you want it, the only thing you would need the account number for is to port your number. You do not need your account number for online access. If you are being asked for the account number, you are likely in the wrong place and trying to setup postpaid online access.

You got a new number with the $300 plan because that plan is only available to new activations, not existing lines. You can’t get the old number back. The only way you could have kept that number would have been to port it out and then port it back so that it would qualify as a new activation.

Thank you for your response sandblaster! (Edited per community guidelines), Is there no other way to get my old number back? And for future reference, how do I port my number out and port it back in? Does it involve any costs?

The only way you can get the old number back is to Port it out to a different service provider and then ask AT&T to Port it back in and replace the temporary number they gave you. You have 30 days to do this from activation of $300 plan

@formerlyknownas makes a good point. Your new number is a new prepaid account. Your old number and account is probably still open meaning you haven’t lost it yet. When was the last time you renewed your $40 plan? Your old account does not get closed until 60 days after your $40 plan expired. You will need to renew a plan on that number in order to port it. If you are already past the 30 days to replace your new number with your old number by porting out and back, another option is to port your old number to google voice. You can then use that number along with your new number.