Dash standby has a maximum of 12 hours

This sounds like a registration problem, the new
handset must be de-registered from its original
base and then registered to your base.

NOTE: the CS6729 handles a maximum of 5
handsets. If you have a handset displaying
"Handset #5" then you have a 'fully populated system'
even if you actually have only one or two handsets.

You will need to clear each handset ( I ) then clear
the base of all registrations ( II ) before re-registering
each handset ( III ) up to the maximum of five.
=====================================
( I ). First, to de-register a handset ---

0. Start by taking off the battery compartment cover.

1. press [menu]
2. scroll up/down to "Settings"
3. enter * 7 8 9 0 #
4. scroll down to "next page", press [select]
5. scroll down to "Register Clear?", press [select]
6. display asks "Confirm?" or "Register Clear??",
press [select]
7. display reads "Ok" and chirps, press [ off ]

8. UNPLUG THE HANDSET BATTERY
(this is why the cover was off)

9. Wait ~30 seconds, re-connect the battery &
replace the cover

10. display reads "Not Registered"
===============================

( II.) To clear the base of all handset registrations
( if needed, else skip ahead to ( III ). )

1. press [Find Handset] on the base and hold
past ~4 sec when the "In Use" lamp lights, at
about 10 secs the "In Use" lamp will begin to
blink. 2. release the [Find Handset] button momentarily,
then briefly press it again. After a moment,
all handsets that were registered to the base
(if any) will display "Not Registered" and be
ready for fresh registration ===============================

( III.) To register each handset to the base:

1. press [Find Handset] on the base and hold for
~4 sec until the "In Use" lamp lights.
2. press # on the handset and set the handset into
the handset cradle on the base
3. within 30 seconds or so, the handset will either
chirp and display "Handset Registered", then
"Handset #x" or it will say "Registration Failed".

NOTE: the battery must be well charged prior to the
registration process, a weak battery may cause a
"Registration Failed". Charge the battery for a
few hours and try the registration ( III ) again.

===============================
Good luck !!

The errant handsets must be de-registered from their
original base and then registered to your base.

NOTE: the CS6649 handles a maximum of only five
handsets. If you have a handset displaying
"Handset #5" then you have a 'fully populated system'
even if you actually have only one or two handsets.
In that case, you can clear each handset ( I ) then clear
the base of all registrations ( II ) before re-registering
each handset ( III ) up to the maximum of five.
=====================================
( I ). First, to de-register a handset ---

0. Start by taking off the battery compartment cover.

1. press [menu]
2. scroll up/down to "Settings"
3. enter * 7 8 9 0 #
4. scroll down to "next page", press [select]
5. scroll down to "Register Clear?", press [select]
6. display asks "Confirm?" or "Register Clear??",
press [select]
7. display reads "Ok" and chirps, press [ off ]

8. UNPLUG THE HANDSET BATTERY
(this is why the cover was off)

9. Wait ~30 seconds, re-connect the battery &
replace the cover

10. display reads "Not Registered"
===============================

( II.) To clear the base of all handset registrations
( if needed, else skip ahead to ( III ). )

1. press [Find Handset] on the base and hold
past ~4 sec when the "In Use" lamp lights, at
about 10 secs the "In Use" lamp will begin to
blink. 2. release the [Find Handset] button momentarily,
then briefly press it again. After a moment,
all handsets that were registered to the base
(if any) will display "Not Registered" and be
ready for fresh registration ===============================

( III.) To register each handset to the base:

1. press [Find Handset] on the base and hold for
~4 sec until the "In Use" lamp lights.
2. press # on the handset and set the handset into
the handset cradle on the base
3. within 30 seconds or so, the handset will either
chirp and display "Handset Registered", then
"Handset #x" or it will say "Registration Failed".

NOTE: the battery must be well charged prior to the
registration process, a weak battery may cause a
"Registration Failed". Charge the battery for a
few hours and try the registration ( III ) again.

===============================
Good luck !!

I think you all totally missed the question. The phone set comes with the base and 2 extensions. He is wanting to BUY a 3rd extension for this unit, if I am reading this correctly. I have the same phone set and I have not found anywhere that sells additional handsets. Sorry.

Silent Mode
You can silence the ringers on the base and all handsets for a specific period of time; the answering system answers any incoming calls without playing anything through the speaker.
With the phone in standby, press and hold # on any handset. The phone prompts you to select the number of hours (1 - 9 or Always On) that you want it to stay in silent mode.
To confirm, press # or just wait about 5 seconds. The phone turns on the answering system and displays Silent Mode On on each handset.After the selected number of hours, the phone automatically exits silent mode. To exit silent mode manually, press and hold # again.
If the phone is in silent mode and you turn off the answering system, the phone exits silent mode.
Multihandset Features To use the features in this section, you need at least 2 handsets.
Your base supports a total of 12 cordless handsets: the one that came with the base and up to 11 DCX160 accessory handsets.You must register accessory handsets to the base before using them. (Handsets that came packaged with the base are already registered.)Handsets that aren't registered display a Not Registered message. For registration instructions, see p. 18, or see the accessory handset manual.If a handset was ever registered to a base, you must reset it before it can register to a new base; see p. 18, or see the accessory handset manual.Conference Calling
When an outside call comes in, two handsets can join in a conference call with the outside caller.
To join a call that's already in progress, just press Talk/flash or SpeSpeSpeaker.To leave the conference call, hang up normally; the other handset remains connected to the call.

Your batteries are either no good, or not charged. When you press the PTT button it takes more energy than being in standby mode. If you have a battery problem it will fail quickly when the PTT is pressed.

Rechargeable batteries last approx. 18-24 months. Check your battery for a 3-digit date code. It is usually a 3 digit number followed by a couple of letters. The numbers tell the date of manufacture. The first number is the year, followed by the week of the year. 025 would be 2010 the 25th week. (or if very old it could be 2000 the 25th week.)

If you feel age is not a problem, you might check your charger(s) to see they are fully powered, and clear of debris on the contacts (and the radio contacts as well). Clean with a pencil eraser if needed.

A few tips: Be certain when placing your radios on the charge they are turned off. Check your wall outlet to be certain it doesn't get shut off with a light switch. Expect to replace your batteries every 2-3 years.

Best regards.

Two to three hours should fully charge your battery, to porlong battery life I would let the battery run as low as possible before recharging. Recharging half charged batteries causes the battery to age faster.

Using a voltmeter, the resisted voltage values for the fan motor are as follows:
LOW- 3 VDC
Med LOW - 6 VDC
Med HIGH - 9 VDC
High - 12 VDC

The fan resistor gets 12 volts from each position of the fan switch, and the 12 VDC is then lowered thru each of the 3 internal resistors inside the fan resistor. My guess is that the resistor has failed, and the resistance wires contained on it may have meshed together. The resistor is on the black box between the blower motor housing and the center of the dash. 2 phillips screws hold it in. Easy to remove/replace. Hope this helps, and thanks for choosing FixYa.

These phones are known for the contacts not transferring adequate voltage to fully charge the phone.

I looked up the battery specs for your phone. They are 3 X 200mah= 600mah battery packs. They should last 8 hrs talk time with the headsets. This style of battery pack in my experience only last about 1 1/2years and sometimes longer and "sometimes" shorter...much shorter. I bet you never had problems when you first got them. Basicly this "battery design arrangment" goes back many years before newer battery technlogy was developed. But designers still insist on using them even today. They are cheap to make. ****Time to replace them with new ones.**** Make sure you follow the mfgs charging instructions: "After battery installation, place the handsets in their base unit or charger and allow to charge for 16 hours before use. You can keep the battery charged by returning the handsets to their base unit or charger after each use. When the battery is fully depleted, a recharge takes about 12 hours. Average talk time on a fully charged battery is about eight hours depending on environmental conditions, standby time is approximately four days." Good luck