The subs never use radio? So why do they come fitted with radio equipment?
ESM may be used to detect, intercept, direction find, etc. any sort of radio transmission. Whether it is a navigation, identification or communication system, its emissions are detectable. It is only a matter of how far and how easily - search google for
low probability of detection or
low probability of intercept and you may find out more.
Any vessel, whether surface or subsurface, will make its detection by enemy easier if it transmits signals - acoustic or electromagnetic (you can also think of chemical, thermal, etc) . VLF radio systems make it possible for submarines to receive information when submerged (there are limits to the depth where it's possible) so that they can remain 'stealth'. However, I suppose that any commander will be happy to use VHF radio while entering a harbour or during Medevac.
These days submarines may be fitted with HF and VHF radio communications systems. Some may use VLF receivers. Some may use SATCOM at UHF or other bands, too. There's a lot of unclassified data available on internet.
How one uses these assets, is another matter. For what we do, which is just having some fun online, I believe the players should agree on the comms methods before starting the game. Forbidding the subs to talk or chat for realism sake will reduce the fun factor. When I hear these guys complain that 'subs don't use radio' I feel like asking them to play a mission where they have to spend three months glued to the computer listening to surface traffic without firing a single torpedo.
We will always have to make a choice between realism and enjoying the game. Combining the two is not likely to work, whatever type of simulation concerned.