Noise on PBX to PBX connection

[Raaif]

Greetings,

I work at a small business where we are using 2 pbx systems, one for our head office and one for our second office. The two locations are about half a mile away. We have connected the two pbx systems through a leased line provided by our ISP. We have a NEC SL1000 at our head office and a panasonic KX-TE at our second office which is connected as an extension to the NEC SL1000.
Quite recently there has been a development of a disturbing noise over the connection. The noise occurs when we dial the extension to the second office from our head office and vice versa.

We called our ISP and they sent a technician to check on the leased line but concluded that the line wasn't the problem. He believes that the noise is caused by the distance between the two offices and that PBX systems does not produce enough power for long distance extensions. However, we haven't had this issue for 2 years, why now?
If what he says is right, what are our alternative options for connecting the two offices?

I apologize if my explanation is too vague, unfortunately i am not tech savvy.
Any help or suggestions would be greatly appreciated.
Thank you in advance for your kind responses.

 

[belevedere]

Can you post more information about the leased line?
Is it an analog line, central office OPX, T-1, IP converted to TDM, E&M TIE line?

 

[Raaif]

It's a T-1 line

 

[belevedere]

Typical noise on digital circuits is a clock slip. If this is a dry pair from Telco then you must have something like an adtran CSU programmed to provide local clocking, or extract the clock from the distant end. If telco is providing the clock, then I would persue it with them.