I know that this is probably a stupid question, but does anybody know a good source for Nokia-style beeps and sounds?
Here is the classic one https://en.wikipedia.org/wiki/Nokia_tune, they have a piano version for cc0 download.
Thou you need to be careful with it, since it is a "sound trademark" and I really don't know if it qualifies for "fair use".
Do you mean DTMF (https://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling)? Aka the tones each number key makes when pressed. If this is the case you don't need to download them you can just generate them with audacity. There is a special DTMF Tone generator included by default.
Not sure if you already had a look at what the resource page has to offer but here is a link to it just in case! https://phillipp.itch.io/nokiajamresources
There is a few options for sounds in the download section!