I prefer the approach of trust.
Some people prefer to work with music. Other people like the freedom to use a personal device to access social media or personal communication (phone calls, texts). As a general policy, unless there are any kind of security or privacy considerations, I think it is appropriate to allow employees to have personal devices at their desk.
Any problems related to people being distracted should be dealt with on an individual basis. If your employees are recording hours and you suspect that they are charging significant quantities of time spent on non-work activities to customer accounts or if your employees are not meeting their commitments, you can deal with those specifically. However, if time is properly accounted for and work is being done, does it really matter?
Of course, this is geared towards leads and managers. If you aren't in a management position, you should simply focus on doing your own work and doing the right thing. If coworkers are doing things that impact their ability to work with the team, then you should raise these concerns to your manager and let them investigate and resolve the problem. If your coworkers are simply distracting you (for example, listening to music so loud you can hear it, not turning their phones to vibrate or silent, their phones are vibrating in a way that you can hear), you should let them know and nicely ask them to do something to remedy the situation so you can work.