First I don't know the frequencies. Nextel will probably have more than 100 frequencies, but that just starts a problem. They will reuse the frequencies several times within the local area as they use directional antennas. In other words, if you knew the frequency, there could be as many as 5 or 6 conversations going on the frequency at the same time. You would pick up just one of them. Then lastly, Nextel uses a digital format called iDEN that your scanner can't decode.
Altel transmissions are primarly on 1.9 GHz, very few in the old cell band. The 1.9 GHz will be in a digital format that you can't decode, even if you could scan the frequency range.
The fact that it is illegal to listen to a cellphone is not the big problem. Modes and in the case of Altel the high frequency is what will stop you.