The only reason I would know the UBU allocation to change is if the auto allocation is enabled, you probably should double check this option. If you issue the command 'off3' check the status of "Auto Allocate UXM UBU" and make sure it is set to off. If not, then enter '6' as the option to 'off3' to disable it.
Even a configuration change such as adding or deleting a line or trunk should not have an impact on this setting as long as the auto allocation is disabled.
The only ways this value would decrease is if someone manually entered 'cnfbusbw' or if somehow the on/off3 "Auto Allocate UXM UBU" was enabled (then disabled again if currently disabled.
