In article %kqsk.741$lf2.630@trnddc07, "Mike"
wrote:

What makes you think the impedence of the ones in the telephones are
different from the ones in your headset?

Because they are two different devices used in two different applications.
All microphones, even electret models, don't have the same impedence and
there are wide variances depending on the application. Even a mic removed
from a DC military headset won't work well in their civilian version because
of the differences in impedence.

What makes you think they are the same?

The voltages and currents they are typically designed to work at.