You could have a look at manual from :-http://www.thephoneresource.com/manuals/ATT_cl2909_manual_i3.pdf
You need to have acrobat reader to view the file. The complete procedure how to change battery is given in that. Once you make sure that the battery is well seated, the next step would be to disconnect the power supply for few minutes, say 15 minutes and then turn it on. If the problem persists, then it is an internal problem.
Then, you could take a cleaning solution and make sure that the battery connectors are fine. Sometimes if the connectors are dirty or bent or damaged - it could cause the same issue. If the problem still persists, either you need to replace the set or open it for repair.When you open the unit, you might find a loose or damaged wire. Other than this, it has to be a faulty mainboard.Honestly, on most occasions repair is expensive as compared to cost of the new unit.
Let me know,if needed further assistance.
Hope i helped you.
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