Do you mean the AT&T CL81301 Cordless telephone? This model doesn't have an answering system. You'd need to adjust the number of rings for your voicemail system through the telephone company's system. See the manual here: https://att.vtp-media.com/media/p/document2/products/%7B40A22BFC-681D-46DC-B836-7CB7C7A691FE%7D/CL81101_CIB_5_610.pdf
If you mean the AT&T CL82301 or the CL83301, first make sure that the system is set to Answer On. Do this with the base Answer On/Off button or through the Menu system (see below for accessing the Answering System menu). You'll need the Answer On/Off line in the handset menu. With the phone system idle (no calls in or out, no one using another handset), press Menu/Select. Use the Up(DIR)/Down(CID) arrows to navigate to Answering Sys. Press Menu/Select. Navigate to Ans Sys Setup and press Menu/Select. Navigate to # of Rings and press Menu/Select. Use the up/down arrows to choose the number of rings and press Menu/Select to Save. Press Off/Cancel to exit the menu system. The manual for this cordless telephone system is available at ManualsLib, https://www.manualslib.com/products/AtAndt-Cl82301-165464.html
. See page 47 of the manual, https://www.manualslib.com/manual/643600/AtAndt-Cl82101.html?page=53#manual
. Note: if you choose Toll Saver, the answering system will pick up after two rings if you have new messages. It will pick up after 4 rings if there are no new messages.
I hope this helps. If you have a different AT&T CL8 * 301, please add a comment with the correct second digit in the model number.