A trunk-to-trunk transfer is also known as an unsupervised conference. The latter term describes in more detail the actual electronic operation of this feature, because it uses a phone system's conference circuits. It also hints toward another fact: In a regular conference the outside parties also have trouble hearing each other.
An acurate analogy of a CO Transfer would be 2 persons trying to converse around a corner in intersecting hallways. The sound has to reach the other person in an indirect way.
Using the same analogy in regards to a CO conference, the two outside parties are still separated by this corner, but the person who used the phone system to initiate this conference, is at the intersection. The two outside callers still can't hear each other well, but they can hear the person in the middle.
Every phone system has this issue, because all phone systems are functionally the same. Some phone systems have a solution to this problem by allowing the technician to adjust the gain between the outside parties. Unfortunately, many do not, such as the Panasonic KXT-A624.