The above solution is woefully incomplete, as it only addresses a single issue.

The "That is not a valid conference number" issue can be caused by any of the following problems:

1. You actually entered an invalid conference number. (doh!)
2. The Zaptel or Dahdi kernel modules are not loaded
3. Zaptel or Dahdi is not configured for your card
4. The Asterisk user does not have access to the pseudo device
5. The Zaptel or Dahdi kernel modules need to be recompiled.
6. Asterisk was not compiled with Zaptel or Dahdi support.

Solution 1: If you entered an invalid conference number:

1. Go to your room!
2. Stand in the corner and contemplate your decision to take this particular career path. You really should be confined to userland considering the fact that you did a search for this solution when the problem clearly originates from a hydrocarbon mass between the chair and the keyboard.
3. Replace the user and try again.

Solution 2: Check to see if the Zaptel or Dahdi kernel modules are loaded:

If you do not have a Zaptel/Dahdi-supported card:

lsmod | grep dummy

If you are using Zaptel:

lsmod | grep zt

If you are using Dahdi:

lsmod | grep dahdi

If you receive a response and see that the modules are loaded, try the next resolution below.

If you do not receive a response, simply restart Zaptel/Dahdi and check to see if it's loaded again.

For Zaptel:

/etc/init.d/zaptel restart

For Dahdi:

/etc/init.d/dahdi restart

Don't have the init scripts? You need to install the tools. If you are compiling from source, see the solution on configuring Zaptel/Dahdi to compile the tools, or just download the complete Dahdi package and follow the solution on recompiling Zaptel/Dahdi below.

Make sure that you're starting Zaptel/Dahdi at system start as well:

For Zaptel:

chkconfig zaptel on

For Dahdi:

chkconfig dahdi on

Otherwise, run the lsmod command again and look for a response.

Yes? Go ahead and attempt the conference/meetme/page again.

No? You either need to configure Dahdi/Zaptel or you need to recompile. See those resolutions below.

Solution 3: Configure Zaptel/Dahdi

This solution is a simple one. Run the following command -

For Zaptel:

For Dahdi:

If you don't have either of those commands (they're typically in /usr/sbin) you need to install the appropriate tools package. Recompiling Dahdi using the solution below includes these tools, however, if you just want to compile the tools, download the appropriate tools from http://downloads.digium.com/pub/telephony/dahdi-tools, extract and compile using the following commands:

Make sure dahdi_cfg is selected

Try your meetme/conference/page again.
No? Try the next solution.

Solution 4: Reset permissions on the Zaptel/Dahdi device:

The user account that is running asterisk requires access to the devices created by Zaptel/Dahdi. If you're running as root, this solution will not solve your problem. Move on to the next one.

For Zaptel:

chown -r asterisk_username:asterisk_groupname /dev/zap

(and /lib/udev/devices/zap depending on your distro)

For Dahdi:

chown -r asterisk_username:asterisk_group /dev/dahdi

(and /lib/udev/devices/dahdi depending on your distro)

If you are using udev, make sure the OWNER= and GROUP= are set correctly in /etc/udev/rules.d/zaptel.rules or /etc/udev/rules.d/dahdi.rules or the next time you restart, you'll experience the same problem.

Attempt the conference/meetme/page again.

No? Try the next resolution.

Solution 5: Recompile and reinstall dahdi/zaptel.

cd /usr/src

if you already have dahdi/zaptel, go ahead and go to that directory. Otherwise, download the latest Dahdi source:

If you have an Asterisk version that does not support Dahdi, the older, deprecated Zaptel sources can be found here: http://downloads.digium.com/pub/telephony/zaptel/releases/

Next, clean, configure, compile, and install the sources:

Doesn't compile and you're using something like RedHat, CentOS 4.2, related and kernel 2.6.9-3? See this bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=180568

Start the service:

Zaptel: /etc/init.d/zaptel start

Dahdi: /etc/init.d/dahdi start

Attempt the conference/meetme/page again.

No? Looks like you need to recompile Asterisk.

Solution 6: Last ditch effort - Recompiling Asterisk

The good news is that if you followed the above sequence, you have the dahdi/zaptel support available to you now. If not, I'll send one of the nuns at the local Catholic school after you with a ruler. Then you'll need to download and compile Dahdi/Zaptel anyway.

So if you did follow the above steps, go to the Asterisk source directory. If you don't have one, follow the good ol' Downloading and Compiling Asterisk guide from Digium.

Otherwise

cd /path/to/asterisk_sourceIf you don't know what that means, follow the first resolution.

Stop Asterisk if it's running.

/etc/init.d/asterisk stop

Now recompile Asterisk:

Head into Channel Drivers and make sure chan_zap or chan_dahdi is not blank.

Start Asterisk back up:

/etc/init.d/asterisk start

Then try your conference/meetme/page again and see if it works.
No? Ya got me. Looks like you'll be sifting through Google results some more. Sorry bout that.