dbb4090d_Internal_Error_Codes___DocWiki__Descriptor
[Code] 21 [Explanation] The maximum number of connections for the leg has been exceeded. An attempt to bridge yet another connection on the leg failed. [Category] 180 [Descriptor] 
Leg connections maxed
Cisco IOS Voice Troubleshooting and Monitoring -- Cisco VoIP Internal Error Codes - DocWiki