Can the antenna of a Wi-Fi modem pick up a cell phone's MIN or ESN?
If memory serves me right WiFi operates at 2.4GHz and 4.8GHz, depending on the exact standard.

Mobile phones on the other hand work in frequency bands varying locally 400-475MHz, 800-950MHz and 1600-1900MHz.

So the answer is "No" you cannot pick up cell phone transmissions with a regular WiFi modem because the frequency bands are very far apart. Apart from that, the protocols (on several layers) used to transfer data are different. 

The different protocols have to do with the difference between transmission (continuous stream of data packets [the hum you hear when you use your phone near a cheap amplifier]) and communication (sending a single packet with data whenever required).


WiFi is related to 802.11 standard
GSM is related to various standards listed here.