There may not be a problem. If there is, changing the phone is generally the only option.
The speakerphones on 6408D+ sets are one-way only speakers. That is, they will take the sound that's the loudest, (either in the room, or coming in from the distant end) and make that the thing they will use.
Consequently, the phone is in either transmit mode or receive mode, but not both at the same time.
They require a bit of discipline to use in a conference call. If both parties begin speaking at the same time, only one will be heard. (Usually the loudest person.)
Background noises can also affect them. Locally, nearby computer cooling fans, overhead ventilation, and other noise sources can cause the speakerphone to switch to transmit when the user want's to hear what's being said by the distant user.
Carpe dialem! (Seize the line!)