Digitakt or Digitakt II? There is great info on frequency values for both as they relate to parameter values here: Digitakt Filter Cheat sheet
Telephone should be in the range of 300 Hz to 3 kHz. If not achievable with the EQ filter type, I think a low/high shelf filter would be the best choice, but those don’t exist on the Digitakt (yet?). I know they are an option on the Syntakt analog tracks. For other flavorings I would lean on bit rate reduction instead of sample rate reduction, sprinkles of overdrive and rebalancing with AMP VOL if you want some distortion or megaphone territory.
Honestly though, I think the simplest solution here is resampling (meaning achieved outside the Digitakt).