After some time your 3088 Dualphone doesn't make the ringtone.
You can't really use that phone anymore. When some one call you, the phone doesn't ring.

It's a common failure for this phone. The phone has 2 speaker and one microphone. You have to replace the rear speaker, the one for the ringing tone.

Here are the steps to open the phone and change the ringtone speaker.

Step 1: Open the Battery Case and Remove the Battery

Open the battery case and remove the battery.

Step 2: Remove the 2 Screws

Use a star screwdriver size 8 and remove the 2 screws

Step 3: Open the Back

Take care when you open the case.
They are 2 extra clips on each side.
Open it slowly, the inside parts are just placed inside.

Step 4:

You can now see the voice speaker on the top right.
And the voice microphone at the bottom of the keyboard.
You can remove the keyboard.
Take a good look. When you'll close the case, everything need to be in the exact place.

Take extra care when you manipulate the device, nothing is screw.
Especially with the screen's frame and the screen.

The ringtone speaker is on the other side.

Step 5: To Open the Other Side

To have access to the ringtone speaker you need to remove all the electronics.
Take care of the 2 charger pins at the bottom.
and take care also of the battery clips.

Then remove gently all the electronics.

Step 6: The Ringtone Speaker

Now you can see the ringtone speaker.

That's the big speaker glued on the back.
It's a 8 Ohm speaker. The diameter is a little less than 30mm. The height is less than 5.5mm.

You can find those speaker in an electronic shop or on ebay.

Step 7: Remove the Speaker

Remove the glue with a sharp knive.

Take the time to remove all the glue.
Then remove the speaker and cut the wire.

Step 8: Replace the Speaker

Place your brand new speaker and glue it.
Put glue on the border, and try to avoid to glue the front vibrating part of the speaker.

Let dry the glue.

Step 9: Solder the Wire.

Solder the speaker's wire. Isolate the whole wire to avoid any short-cuts.

Step 10: End

Put back everything in the box. Read the previous steps in the reverse order. And put back the battey.
Now your Dualphone 3088 is repaired.

You can check in the phone's menu by changing the ringtone.
And also call the "echo/sound test" service to check that your phone fully work.