Here's my _guess_ as to why the pitch of the "beep" in the telephone
receiver appears to increase as the earpiece is moved away from the
ear.

I think it has to do with the quality of acoustic coupling.  When
the earpiece is in contact with your ear, there is maximum acoustic
coupling.  An impedance match, so to speak.  When it is moved away
there is very poor coupling.

The "beep" is a complex sound (not a sine wave) containing many
frequencies not unlike the overtones/partials of a piano string.  When
the coupling is good, everything is transmitted.  When the coupling is
poor, the high frequencies travel better than the low and so the higher
frequencies are heard better than the lower ones giving the impression
that the pitch rises.

Jim Heyworth,
Sechelt, B.C., Canada