The Uniden XDECT 8315 does not have an answering machine. It will check your voice mail. Setting that greeting depends on your telephone service provider. Check your provider's voice mail FAQ.
To have your XDECT 8315 get your voive mail messages, press Message and then select Voice Mail. Then follow the prompts to enter your voice mail access number. Alternatively, you can press Menu > Global Menu Settings > Edit Voice Mail. See pages 17 and 32 of the manual. (See the link below.)
The manual for the XDECT 8315 is available on the product page: https://uniden.com.au/product/xdect-8315-1/
. The current link to the manual is https://uniden.com.au/wp-content/uploads/2017/06/XDECT_83151_OM.pdf
If you do not have voice mail from your telephone company, you can add a stand-alone answering machine. Just make sure to connect the answering machine to the telephone jack. Then connect the answering machine RJ-11 out to the telephone's RJ-11 jack on the base.
I hope this helps.