Yes, 411A - $800 and a small weight penalty on your small aircraft cabin. What about shielding from interference from any cabin position on a 767?
P.S. - how much did your users irritate the phone company? If they were dialling from the air they would be covering several cells, and apparently it is difficult for a cellphone network to cope with large numbers of airborne phones. Also in the UK I believe it is against some phone contracts, as well as being illegal.