alternatively, if you really didnt want to pay for an extension to be installed, you could purchase a gsm/ptsn converter, slip in a sim card and stick that next to the panel. pull the wire from the old socket, crimp a plug on the end and away you go. ADT none the wiser!
this is a crazy, **** about face, probably more expensive route to take but still a solution
personally, i would have offered the ADT engineer a sneaky £50 to route a cable through the house for me quickly. would have been a much neater install, job done whilst he was on site, no need to pay for a return visit, everyones a winner
of course this is frowned upon these days, how dare an engineer help out a returning customer!!