The IC-T81A may be blocked from direct entry of cellular frequencies, but you can still receive analog cellular frequencies on it quite nicely. All you need is a little arithmetic...
If you've performed the mods listed on this site to open up the receive range to 999.95MHz, cellular is no problem. All you have to do is dial in the image frequency for cellular. That is, dial in a freq that is the cellular freq minus 2 times the IF frequency for the T81A, which is 69.4 MHz.
The cellular band (for analog phones only) from handheld to cell site is 824-849 MHz. Transmit back from cell site to handheld is 869-894 MHz. (There are a few control and data channels interspersed in those ranges too.) That corresponds to 685.1 to 755.1 MHz display on the T81A.
So, for example, to receive transmissions from a cellular phone at 826 MHz, just set the T81A to 826 - 138.9 = 687.1 MHz.
Towers Output (display on T81A 730.1 to 755.1 MHz) to RX 869-894 MHz
Cellular Input (display on T81A 685.1 to 710.1 MHz)to RX 824-849 MHz