Green’s Dictionary of Slang

a telephone.

1959		J. Gosling Ghost Squad 25: Thieves’ argot, spoken properly, is a foreign language which needs to be learned [...] a phone can be either ‘trombone’ or ‘Eau de Cologne’.
1992		R. Puxley Cockney Rabbit.
2011		J.J. Connolly Viva La Madness 112: Sling that trom — I’ll get you another one.
2011		J.J. Connolly Viva La Madness 116: He double-checks that I left the trombone at the hotel. I tell him I no longer possess a mobile phone.