I just bought a pair of wireless headphones that broadcast at 900Mhz. Say I have a wireless phone that also broadcasts at 900Mhz, is it possible for me to pick up the signal from it on these?
No. Although both say 900 MHz on them, the cordless phones and the headphones operate on different frequencies, as assigned by the FCC.
To be specific, cordless phones operate on one of 80 channels between 902 MHz - 928 MHz and headphones operate on one of three channels, either channel 1: 913.5 MHz, channel 2: 914.0 MHz, or channel 3: 914.5 MHz. Although the headphones fall within the range of the cordless phones, there is no sharing of channels.
Here is a list of 900 MHz phone channels