There may be no easy answer---as I have noticed in other threads. I just solved my problem today based on another post so I hope it helps you as well. I have a HTC One E8 through Sprint. I called them, went to the store and to the repair store but no one knew anything. The online user manual used the MIM (number code) *72. Then instructed to follow that with the 10 digit phone number and press "Call". It did not work--recorded voice stated invalid number or code. This is what I did based on the other thread. Call Forwarding: *7215555555555#. It has the *72 followed by 1 then the 10 digit phone number you want to forward to then #. It worked and I heard the verification tones. To cancel Call Forwarding: *720#. It worked as well. Conditional Call Forwarding: *7315555555555#. This will allow a call to ring on your phone for about 15 - 20 seconds then forward the call. To cancel Conditional Call Forwarding: *730#
Since I am leaving town for a week (with limited service) and my phone is my business phone, I need a solution to forward to an associate. I am forwarding to an online service that will log the calls and email me the voicemails. An interesting note is the * didn't appear on my phone when I originally tried to activate the call forwarding. It wasn't until I added the # on the end that it appeared when I hit the "call" button. I hope this helps, my friend.