Unfortunately, there appears to be an issue with your specific account.
We recommend calling Sprint Technical Support and stating the following:
"I am unable to activate conditional call forwarding on my number. Dialing the star seven four (*74) activation code does not work. Can you activate my conditional call forwarding for me? I want it to forward calls when busy"
When they ask what number you would like your calls forwarded to give them this number: (559) 533-5694
Please don't mention Assistant, your carrier might state that they do not support third-party applications as a way to avoid dealing with the request.
Note: If the representative is unwilling or unable to help you, ask to speak to another representative or supervisor. Do not let them tell you that they are not able to activate your conditional call forwarding. Call forwarding is a carrier side setting and they alone have the ability to manage it.
Sprint Customer Support
Phone number: (888) 211-4727