You can't use a 2.4 GHz phone and Airport together.
Airport uses the 802.11b (Airport Extreme uses 802.11b and 802.11g) and there are only 3, count them 3, non-overlapping channels in the 2.4 GHz range.
So, it will be very unlikely that you will be able to get the phone to not cause interference on your airport network.
Most work environments that have 802.11 networks (that I know of) forbid the use of 2.4 GHz phones because they create such a large area of high power interference. They are too disruptive to the network that it is easiser to ban them than to attempt to get them to work together.
So, your best be is to go out and get yourself a 900 MHz phone. (Plus according to the laws of physics, 900 MHz phones have longer range than 2.4 GHz phones)
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