I keep my phone on the silent mode.

[tufguy]

1) I keep my phone on the silent mode.

2) I keep my phone in the silent mode.

Please check my sentences.

 

[tedmc]

on silent mode.

 

[Yankee]

on silent mode.

OK, but I have heard/used "in".

 

[emsr2d2]

I've never heard or used "on" in that context. I wouldn't even use "mode".

My phone is on silent.
Please put your phones on silent.

 

[Tdol]

OK, but I have heard/used "in".

I haven't heard it used in BrE.

 

[Rover_KE]

Also 'I keep my phone on vibrate/silent'.

 

[SoothingDave]

"In" or "on" works for me, but I prefer "in."