CallManager provides a software-based conference bridge service, and I'm going to proceed on the assumption that you haven't accidentally deleted it. I'm also going to assume you don't have any hardware conference bridges, but I'll come back to that assumption later. Let's double-check your software bridges first:
Go from the main ccmadmin webpage to the Service menu, then Media Resource, then Conference Bridge. The path to this is slightly different on versions of CallManager below 3.3, but you should be able to find it easily enough. Make sure that you have one or more conference bridges on this list. If you're running CallManager 3.3(3), you will have to hit Find with no parameters to get the list to show up. Each listed bridge should have something showing up in the Status and IP Address fields. If they are missing, that bridge is not operational.
You probably have one conference bridge for each CallManager you operate, so if you have a cluster of two, you should have two. And they're probably both operational. I would suspect you've been working with with media resource groups and media resource group lists or device pools and these conference bridge(s) have been made unavailable to the devices you're trying to conference together.
To start off, here's some documentation on media resource groups and media resource group lists:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00800c4c69.html
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00800cde78.html
And documentation on device pools:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00800c4bad.html
To make a long story short, you need a MRG that contains your conference bridge and any other services like transcoders or music-on-hold servers that you want these devices to have access to. Then you place that MRG in a MRGL, which is an ordered list of MRGs. This is useful if you have many different bridges to choose from, but you probably have just the software ones on CallManager. Now you have a MRGL which you can apply directly to one or more phones to give them access to your conference bridges.
However, the clean and correct way to do this is usually via a device pool, which can be viewed as "presets" for a pool of devices. You can define a bunch of variables once in a device pool, and assign just the device pool to the phones in a given group. In this way you can change the MRGL (or any of a number of other settings) on the entire list of phones assigned to that device pool at once. So, if you're using device pools or want to use device pools, assign the MRGL to the device pool and leave the MRGL field blank on the phones.
If your configuration is more advanced and you expect to have hardware conference bridges (usually IOS routers or Catalyst switch add-on blade) at this site, include those in your MRG and MRGL in the order that you want to use them. Create more MRGs and MRGLs as necessary to define ordering rules for more sites if you have different resources local to each that you want to use first.
If you have bridges that aren't showing up as registered, or otherwise have got the bridges correctly assigned to the phones, then we'll need you to further clarify the problem in another post.
