3 of my 5 phones say "No Line"?

This sounds like a registration problem, the errant
handset(s) and the base have lost identification
with each other.

NOTE: the CS6429 works a maximum of 5
handsets, so you have a "fully populated system".

You will have to clear each handset ( I ) then clear
the base of all registrations ( II ) before re-registering
each handset ( III ).
=====================================
( I ). First de-register each 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

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 !!