I know i'ts too late. But I post my method of forwarding calls with NFC tags. Not shure with Google voice or other, but works with SIM cards.
1. Download "Trigger" app
2. Download "Call Plugin for Trigger"
3. Download "Tag Re-use Plugin for Trigger"
4. Start "Trigger" and make a new task
5. In first window press + and add NFC, tap Next
6. In second window name your task (I named "Call Divert ON"), tap +, choose "Phone" -> "Make a call"
7. Type in MMI code which is provided by your operator for call forwarding. I think usually you can find it on their webpage on you can call them. (Mine was **21*phonenumber# ). Click add.
8. In the next frame do the same thing, but type number to deactivate call forwarding. (Mine was ##21# )
9. Click next, and scan your NFC tag.
From now on if you scan NFC tag it will activate MMI code and activate Call Forwarding.
If you scan NFC tag again it will deactivate call forwarding.
To deactivate call forwarding I allso put trigger widget onto one of the desktops. Just in case you leava a car and forget to deactivate call forwarding.
Also try playing with Trigger commands. I also put "Tunr off screen after 15 secs" on first task, and "Turn off screen after 2 minutes in seccond task" . But had to ass pause in first to exclude nfc rescanning.
Keep in mind that usually NFC will work only when phone is on and unlocked. To make it run even in sleep mode you will have to root phone and run modules with Xposed or something else.