I was recently denied phone visits. What the criteria they use to make that decision.

It might be on your end or on the inmate's end. If you were on a call and said something that caught the ear of a CO and reported you. If you attempted to add another person to the call while it was going (third-party calls are a no-no), that might do it. Or, the inmate is on some disciplinary period for violations inside. Regardless, you can call or write the warden and ask what specific rule was broken to be denied calls.