Without the exact model it is difficult to find the manual.
Here the manual for 900 series/996:Uniden EZI996.
I also quote from manual:
Selecting the Ringer Tone and Volume
Base ringer on/off
The ringer switch allows you to set the base ringer on or
off. If you set the switch to off, you never hear the base
Base ringer tone
Press the ringer tone button in standby mode to select one of 7 ringer tones.
Base ringer volume
Slide the ringer volume adjuster to set the base ringer volume.
Handset ringer tone and volume
Press the volume up key or volume down key (labeled as the volume/ /+ or volume/ /- key) in standby mode to select one of five ringer tones and volume combinations