  Gravitational-wave echoes in the post-merger ringdown phase are under intense
scrutiny as probes of near-horizon quantum structures and as signatures of
exotic states of matter in ultracompact stars. We present an analytical
template that describes the ringdown and the echo signal for nonspinning
objects in terms of two physical parameters: the reflectivity and the redshift
at the surface of the object. We characterize the properties of the template
and adopt it in a preliminary parameter estimation with current (aLIGO) and
future (Cosmic Explorer, Einstein Telescope, LISA) gravitational-wave
detectors. For fixed signal-to-noise ratio in the post-merger phase, the
constraints on the model parameters depend only mildly on the details of the
detector sensitivity curve, but depend strongly on the reflectivity. Our
analysis suggests that it might be possible to detect or rule out Planckian
corrections at the horizon scale for perfectly-reflecting ultracompact objects
at $5\sigma$ confidence level with Advanced LIGO/Virgo. On the other hand,
signal-to-noise ratios in the ringdown phase equal to $\approx 100$ (as
achievable with future interferometers) might allow us to probe near-horizon
quantum structures with reflectivity $\gtrsim30\%$ ($\gtrsim85\%$) at $2\sigma$
($3\sigma$) level.
