Some countries have telephone networks that list multiple impedance requirements. When using the Cisco ATA 186 for voice applications, it is important to closely approximate the impedance of the typical handsets used in the region when selecting the proper configuration. The incorrect choice may lead to poor echo cancellation performance. The ATA186-I1 unit comes with standard 600ohm impedance for each RJ-11 FXS port. The ATA186-I2 comes with standard complex impedance for each FXS port (270ohm in series with 750ohm and 150nF in parallel).

The difference is that the Cisco ATA186-I1 and Cisco ATA188-I1 provide 600-ohm resistive impedance. The Cisco ATA186-I2 and Cisco ATA188-I2 provide 270 ohm + 750 ohm // 150-nF complex impedance? which in summary means that the Cisco ATA 186-I2 has a better impedance coupling than the Cisco ATA 186-I1.

The impedance coupling feature matters mostly for faxes that need to have their impedance coupled in order to work fine.

Doing some research in our DB most of customers from Colombia were using ATA186-I1

Per my experience I1 is mainly used in the North and South America and some Asian countries.

HTH