You are dealing with two completely different issues and are getting bad advice from someone who doesn't understand this.
PRI rides on an underlying T-1 digital circuit. This is carried as 4-wire, one pair in each direction. The signal level of the digital carrier can be adjusted by using line build-out (abbreviated LBO) on the transmit side to ensure that it is of sufficient level to drive the digital receiver. If the signal is too low, noise will cause bit errors. If too high, overload of the receive electronics can also cause errors. However, this has absolutely nothing to do with the echo. If your T-1 interface on the PBX isn't showing errors, tweaking the LBO won't help the performance and could make it worse.
Keep in mind that PRI/T-1 is digital. As long as the line isn't showing errors, the level of the incoming digital signal won't matter at all with regard to the analog signal level. The analog signal level is set by the codec on the analog line card of the PBX, not by the line build-out on the digital signal.
To control echo, you'll need to look at the configuration of the PBX analog line cards. Transmit and receive gain are adjustable there. Try lowering one or both. Usually running the analog gain too high results in echo at the far end, not heard by your customer but by the other party. Also check the line card configuration to make sure that they aren't optioned for fax or modem use. There are normally echo-cancelers built in for analog voice use, but these are typically disabled for fax and modem use. If analog lines are shared for voice and modem/fax, some equipment monitors for fax/modem tones and disables the echo-cancelers for data/fax only but leaves them in for voice.
In your situation, I'd look for a more knowledgeable PBX technician.
