If DECT signal strength is high enough it can interfere with 802.11b, generally because many DECT phones don't attenuate enough outside the working band (900MHz) even though 802.11b is up at 2.4GHz.

As Robricc says though, you should be able to find a range which is less susceptible, and most vendors of decent 802.11 kit provide interference/monitoring tools which should let you find the frequency range of the interference so you can avoid it.