updated plan, now 2 lines wont work

Prepaid or postpaid plan?  Changing prepaid plan terminates current plan and requires immediate payment on the new plan. Link below on changing plans. 

https://forums.att.com/conversations/plans-features/i-think-i-need-to-refund-when-changing-to-another-prepaid-plan/63b5086dc1c54d548b8bcbaf#

I believe they were both postpaid plans. Went from family talk nation 700 with rollover to ATT unlimited extra EL. Just spent 1 1/2 hours on hold with customer service to get disconnected before even talking to anyone.

Hi @DankjsdfjksdfkjhdrfskhdrfvsdvkDFSDFShkDFS, we understand how you feel and we want to help you get this sorted out. 

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How old are your phones and SIM cards?  Do you have 5G SIMs?  You may need to go into a store to get updated SIMs.

the phone in question is only about a year and a half old but I'm leaning toward it needs a new sim card. Can I get them same day at a store? 

Updated SIM cards would have been required more than two years ago if that were the problem. I very much doubt that is the problem. 

Thanks for all the help everyone. I stopped into my local att store. They gave me a new SIM card and activated it. Phone works again. Problem is resolved. 

family talk nation 700 with rollover

Holy crap! I'm surprised AT&T didn't kick you off that plan years ago!

I moved off the Nationwide 450 plan without text years ago when it became more expensive than the unlimited plan with text at the time. I did not have to change my SIM in the iPhone 6 that was moved over to my iPhone Xs without any problem.