Hello, I have a Uniden PowerMax5.8GHz digital answering machine with two cordless phones. The problem I'm experiencing is the base phone is not working properly. The caller id is not working and the screen is blank. Also when I try to deregister one of the handsets, it states that the base in unavailable.

The Uniden DECT1560 isn't an answering machine. Please consult the documentation for whatever answering machine you have connected to your phone line.

What model Uniden cordless phone?

Since you neglected to specify the model of your Uniden 5.8GHz cordless system I can only suggest you select it from the list beginning here to download the manual.

Add more handsets to what model Uniden phone?

I believe you use the "memo rec" In the silver color on the base, second line of buttons on far left.

Try a new battery. That is usually what "88" means.

Try going here
http://www.mycusthelp.com/uniden/

unplug main base for a minute then put each phone back in main base

If the handset has previously worked with the base - press the hangup button and the # key at the same time on the handset for five seconds and follow the prompts to deregister the handset. (System reset - deregister HS - which handset) It will tell you that the handset is deregistered. Then put the handset onto the base and it will register the handset.

You can go to Uniden.com to see the manual for these phones.