In your system the volume is probably set and adjusted in ARTD route data. Although this setting is specified for ODT and DTI types of trunks, it may still function on COT's.
CDN 30 - PAD: This data determines Pad control on the ODT/DTI circuit card,see Note (CDN = 30)
ODT DTI
0: No Pad 0: 8 db Pad
1: 2 dB Pad 1: 2 dB Pad
2: 4 dB Pad 2: 4 dB Pad
3: 12 dB Pad 3: 6 dB Pad
4: 16 dB Pad 4: 8 dB Pad
5: Not used 5: Not used
6: Not used 6: Not used
7: Pad Off
Note: CDN30=0: PAD value can also be set by switch setting on the ODT /DTI circuit card.
CDN30= 1 ~ 6: Actual PAD value depends on PAD ROM on the trunk circuit card. (See the “Circuit
Card Manual”.
I believe a card reset is required, or all circuits need to go idle for any changes to take effect.