I thought hawks was talking about analog trunks.
To start with, I should have asked what type of trunks are used at that site. A matching signaling type (fxs_ls, fxs_gs or fxs_ks for loop start, ground start and kewl start) must be set in the file chan_dahdi.conf in the channels section. Disconnect supervision is then enabled automatically based on the type.
If the CO generates a disconnect tone, then and only then the change of the RX gain could play a role.