Handset will not work

Have you tried another handset or especially another handset cord? Those problems are usually poor connections at the small modular connectors for the handset.

need help desperately., certain keys on my phone does not work and I can not go back to the main screen with back button key. What should i do?

Does the SPEAKER button light up red when you press it?
Does it work on one handset but not the other? Or does it not work on any handset?
It's a relatively common problem with these phones. It's more than likely broken...
http://gigaset.com/medias/sys_master/A31008-M2200-R301-5-3S19_en_US_CA.pdf

There are two things to check that you can fix. First, try replacing the handset (curly) cord. A broken wire in the cord can make this happen. If that's not it, watch the hookswitch. That's the plunger that goes down when you put the handset in the cradle, and pops back up when you lift the handset. If it's stuck down, you will only get speaker and no sound in/out of the handset.

Most likely, the switchhook button (under the handset) is in need of repair. Have a qualified repair shop repair or refurbish the phone.

Check the bottom of the set, and make sure that the handset is plugged into the handset - not - handset to the headset jack.

Make sure that the cords that attach the handset are all plugged in.

Take someone elses handset and test to see if it the set or maybe you have a bad handset issue.

Good Luck

Simply pick up the receiver

Well ~ eagleeigh731 has it mostly correct, do what he said to do and get into the programming --> find SYSTEM PRGRAMMING --> hit next until you find CHANGE DNs--> OLD DNs (this is the old extn#) 222 then type in NEW DN 431--> then hit the end or release button.
Voila your new extension is now there... to check dial FEATURE 0 then press the intercom button, the new extension should be there.

Look at the bottom of the set that is giving you troubles, check to see if the handset is plugged into the correct spot.

It should be plugged into the Handset jack not the headset jack.

This is a common mistake.

We would like to change the name displays on our phones