Green’s Dictionary of Slang

the telephone.

1979		R. Barker Fletcher’s Book of Rhy. Sl. 21: I’d just come out the battle / And was looking for a dog.
1983		A. Payne ‘Senior Citizen Caine’ Minder [TV script] 57: Get on the dog and invite old Arthur down for an evening on the river.
1999		Guardian 12 Nov. 35: I’ve been trying to talk all week but your dog’s been switched off.
2000		Guardian 14 Jan. 32: It is, of course, Rodney on the dog.
2007		S. Maloney Sucked In 230: I gave it [i.e. a plant] a drink and hit the dog. ‘Yes, what now?’ barked Inky, answering his phone after one ring.