Verizon offer the option to turn call forwarding on or off online, but it requires a code to be sent the phone. If I had the phone, I wouldn't need to be activating call forwarding online. There should be an alternate way to receive the code. Maybe via email?
Have you looked at https://www.verizonwireless.com/support/how-to-use-call-forwarding/? You may be able to send the code to another line if there is one.
It looks like if Call Forwarding has been set up this way before, a text to confirm it is not required. I, for one, am going to go thru this Call Forwarding set up with a confirmation text to establish it ahead of possibly losing my phone.
Possibly a call to Customer Service will allow you to confirm your identity and get call forwarding activated (you might have to borrow a phone).
From the website Call Forwarding page: