20 Most Recent AT&T 1738 Digital Answering Machine Questions & Answers

http://telephones.att.com/docMgt/public/art/21714/1738_english_manualpart2.pdf is were you can get the owners manual for this device.

contact your local sheriff's forensics unit and, they can advise you retrieval requires expertise and specialised equipment

I think it may be to late but what you can do for furture reference is download the ATT Messages app that will send you all messages like text, voicemail to that app just in case it gets accidently deleted. You might try to contact ATT to see if they can pull up the data for you.