A cheap old mobile phone and a T91 remote would no doubt come to about £30 all in and just require some wiring work.

Wire from the phones ringer speaker to the button on the T91 and have them both powered thru a 12v accessory socket adapter(they would draw very little power long term in a rangey) and jobs a good en

Or fit a much bigger antenna to the FBH remote pack instead  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

took a couple of snaps of GSM unit
no soldering fiddly wires to old cell phones(you still need a T60/T90 remote and receiver for this)