1…I can call whenever or however it is convenient to you
I think : " I can call you at your convenience. "
“I can call whenever it is convenient to you.” = “I can make a phone call at any time it is convenient to you.” is fine. It is a little different from “I can call you at your convenience.” = “I can give you a phone call at your convenience.”
Both are acceptable, though Haihao’s version is definitely more precise.