If its for signals rather than power, I use 22 or 24AWG 7 to 9 strands PVC insulation 'hookup' wire. 1mm I feel is far too large and in-flexible. Nothing more frustrating than trying to debug the project where wires break off because they are too stiff at the solder joint. Same reason I would not propose ethernet wire – they have only 4 or 5 thickish copper strands and break easily at the solder joint after some flexing. If you are sure of your wiring, and the board won't be manipulated, then I suppose stiffer wiring is OK, but that's tempting Murphy..
Duncan, here in Namibia, 'phone cable is single solid strand – I would not recommend that!
Joe