OKI ADPCM is widely used in the CTI industry because it is the principal format supported by Dialogic. As the market leader, they tend to define "common practice". It offers a good balance of simplicity and quality at rates of 24kbps or 32kbps. 32kbps is obtained by ADPCM compressing 8k samples/second linear PCM. 24kbps is obtained by resampling to 6k samples/second and using the same ADPCM compression algorithm on the slower samples.
The algorithms for this ADPCM codec can be found in "PC Telephony - The complete guide to designing, building and programming systems using Dialogic and Related Hardware" by Bob Edgar. pg 272-276.