How do you replace the answering machine greeting?

you have not bothered to give the make and model of your phone so this answer is relevant to uniden dect 1635 but will be a guide to your unit as well as most use the same format
recording a personal greeting (2 to 30 seconds long)
1--open the menu with the phone in stand-by
select answering setup, then record greeting
2--press menu/select to start recording Wait until the system says --"record greeting" before speaking
3--press menu/select to stop recording. The system plays back your new greeting
4--- to keep the message press end
to re-record it press menu/select
in your manual look for the section " using answering system" or "personal the greeting"

Here is the manual for your phone. You can download the manual and the instructions for setting up your answering machine are on page 43:

http://www.uniden.com/pdf/DECT1580-3om.pdf

Setting Up the Answering System
You can set up your answering system's features from the handset. Only one handset at a time can access the answering system menus.

Note: If you have multiple bases, select the base with answering system you want to access (see page 15).

Recording a Personal Greeting
Your answering system comes with a pre-recorded outgoing message or greeting that plays when you receive a call: "Hello, no one is available to take your call. Please leave a message after the tone." You can record your own personal outgoing greeting. Your recorded greeting must be between two seconds and thirty seconds long.

Press /mute. The system announces the number of new and old messages, then starts to play the messages unless it has no messages.

Press 5 to stop the announcements or message playback. You will hear intermittent beeps indicating that the system is in the command waiting mode.

Press 8 to start the recording. The system announces, "Record greeting." Begin recording after the announcement.

When you finish recording, press 8 again or press 5. You will hear a confirmation tone, and your recorded greeting plays back for you.

Selecting a Greeting

Once you have recorded a personal greeting, the phone automatically switches to your personal greeting. You can
also switch back and forth between the pre-recorded greeting and your own greeting at any time.

Press /mute. The system announces the number of new and old messages, then starts to play the messages unless it has no messages.

Press 5 to stop the announcements or message playback. You will hear intermittent beeps indicating that the system is in the command waiting mode.

Press 6. The system plays the current greeting.

Hi Ken.

Here the manual for Presidian 43 series.

Regards.

Ginko.

Recording your greeting message You can record your own greeting message (2 minutes max.). 1 {GREETING REC} 2 Within 10 seconds, press {GREETING REC} again. 3 After the base unit beeps, speak clearly about 20 cm away from the MIC. 4 Press {?} (STOP) to stop recording.
Playing back the greeting message Press {GREETING CHECK}.
You can also operate the answering system by pressing dial keys on the handset
Key Operation {^} or {V} Adjust the receiver/speaker volume {1} or {<} Repeat message (during playback)*1 {2} or {>} Skip message (during playback) {4} Play new messages {5} Play all messages {8} Turn answering system on {9} Stop playback*2

Simply hold down to greeting button for around 5 seconds and it will tell you to record you new greeting. To stop greeting recoding after you finished speaking then just push play. Listen to your new recorded message.

Greeting message
When the unit answers a call, callers
are greeted by a greeting message. You
can record your own greeting message
or use a prerecorded greeting
message.
Recording your greeting
message
You can record your own greeting
message (max. 2 minutes and 30
seconds). If you do not record a
greeting message, a prerecorded
greeting message is used.
1 {MENU}
2 {V}/{^}: “Answering device” i
{OK}
3 {V}/{^}: “Record greeting” i
{OK}
4 After the handset beeps, speak
clearly about 20 cm (8 inches) away
from the microphone.
5 To stop recording, press {OK}.
6 {OFF}
Using a prerecorded greeting
message
If you erase or do not record your own
greeting message, the unit plays a
prerecorded greeting message for
callers and ask them to leave
messages.

Settings:
In Settings dialog you can set the following options for Best Answering Machine:

* Greetings - allows setting up the greetings played on pick-up;
* Active in profiles - defines in which profiles the application will pick-up the phone and record messages;
* Storage - allows selecting on which disk the records will be saved;
* Pick up after - sets the time after which the call will be answered;
* Record time - maximum amount of time the message is being recorded;
* Format - message file format (amr,wav);
* Icon - configures the visibility of the icon displaying application activity and presence of new messages; depending on this parameter, can be shown in standby screen only (Phone application) or in any application;
* Play volume - play volume for the greeting;
* Records lifetime - automatically removes old messages;

Messages:
In this view you can listen to new and old messages, delete unnecessary messages, send the recorded messages and call the person directly from this view.

During the call:

When Answering Machine is processing an incoming call, you will see a dialog showing the application's progress with the ability to stop it:

* Answer - when this command is chosen during a call, it stops greeting playback or message record and you can continue talking to the caller;
* Hang up - stops greeting playback or message recording and terminates the call; hope this will be helpful...

enter answ.mach setup menu
then record greeting submenu
press start soft key
youll hear start recording mess
you have 30 sec v countdown to do message then it automatically uploads

#1 Try changing the 9Volt Battery
#2 Hold down the greeting button,wait a couple of seconds and record your message