Phones display power failure or out of range when turned on.

Did you connect another phone to make sure that wiring is not a problem? Because you visually check it, doesn't mean that there is not a problem.

To set Unconditional Call Forward ON Dial "*72" for a dial tone. Dial the forwarding number followed by "#". Wait for dial tone. LCD will display "Call FWD Activated"
To Cancel Unconditional Call Forward: dial "*73" and get the dial tone, then hang up. LCD will display "Call FWD Activated

but, since you have tried those two, it could be that there might still be a different type of call forward option that is still ON, on your phone like Busy Call Forward and Delayed Call Forward 1. To Cancel Busy Call Forward: dial "*91". Wait for dial tone. Hang up.
2.To Cancel Delayed Call Forward: Dial "*93" for a dial tone, then hang up.

Let me provide you a link to the website which I used as a reference for your future reference.
http://www.grandstream.com/user_manuals/GXP_User_Manual.pdf

I can't get dial tone either. Let me know if you find a solution please.

Sorry, this is not a solution, but I have the exact same problem! Did you ever figure it out??

get a new phone

The steps are outlined in the article below:

D900 FAQ – How do I disable or mute the keypad tone on my SGH-D900? admin ⋅ March 6, 2009 --> Note: In order to disable the keypad tone, the Dialing Display Font Type must first be set to “Normal”
To change the dialing display font type follow the steps below:

1. From the standby screen, press the left soft key for Menu
2. Select Settings (9)
3. Select Display Settings (3)
4. Select Dialing Display (6)
5. Select Font Type (1)
6. Highlight either Normal or Theme, using the navigation keys
7. Press the left soft key to Save the Dialing Display Font Type setting

To turn off the keypad tone follow the steps below:

1. From the standby screen, press the left soft key for Menu
2. Select Settings (9)
3. Select Sound Settings (4)
4. Select Keypad Tone (2)
5. Highlight Off, using the up navigation key
6. Press the left soft key to Save the Keypad Tone setting

What all you need can be decided upon after eliminating the faulty items one by one. First the AC adapter. Check with another device if the adapter is functional. Or if you have a multimeter. Next the phone, try another adapter. Where from? Read the specs printed on the adapter. It will tell you the Input and output voltages. Then it also tells you about the frequency. Then the wattage. If all of them match or come close you can use it. Trip to the store is warranted. Thunderstorms and lightenings generally do not leave anything intact. So, I hope your phone is intact. Adapter can be replaced easily.

what does it mean what your phone is out of range