what is the meaning of these errors?

I added two IMEI iPhone 6s of AT & T but in both receiving error messages. I wonder why and how to solve them.

447097

Rejected

At&t Usa - IPhone 6s/6s+/6/6+ Clean Imei Only (Fast) 35325407xxxxxxx
2024939048.Denied.Unpaid-bill446995

Rejected

At&t Usa - IPhone 6s/6s+/6/6+ Clean Imei Only (Fast) 35923306xxxxxxx 2024938248.Denied.IMEI_ACTIVE_OTHER_ACCOUNT

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what is the meaning of these errors?