[cisco-voip] ATA ohm impedance?

[Dane]

Can someone enlighten me as to what is "ohm impedance" particularly in
reference to the Cisco ATA's ?

Have to replace an ATA and was looking up parts and noticed there are
two in which to choose from:

Cisco ATA 186 with 600 ohm impedance
 ATA186-I1-A

Cisco ATA 186 with complex impedance (270 ohm in series with 750 ohm
and 150 NF in parallel)
 ATA186-I2-A


However I can't say I am familiar with the differences.  Would
appreciate if anyone could summarize or brief me on this.