Rats can hear up to 80 or 90 kHz, and can emit noises up to ~50 kHz.

Actual recordings of rats with specific products

It seems like the Magenta Bat4 is popular among home users.

If you're looking for a frequency-division style device, the BatBox Baton is relatively low-priced.

What different sounds mean

This paper explores the different ranges pretty well.

Phonemes that are good for commands or names

Once I got my bat detector, I noticed that the noises my cats have been responding to all along were noises that produce some sound into the 30-50 kHz range at least. (eg. hissss, kissing noises, rubbing two bits of skin together)

After experimenting, I found that phonemes that generate the most ultrasound are the fricatives. The stop phonemes often have some ultrasound as well, but not as strong and not reliably. (eg. "map", "kiss")

I wrote a program to find words that have a high number of ultrasound components. I then spoke them through the bat detector, and selected ones that had a fairly unique signature in the 30-50 kHz range. (a unique rhythm, or a unique series of fricatives+stops).