Highly operator specific. Many operators require drivers to have access to a phone for operational purposes.
In every case, its unacceptable to be messaging/calling/scrolling whilst driving a train, however, beyond this its all different.
For instance, many operators will encourage the use of technology. At my place, I'm allowed to use any mobile device in the cab provided its not connected to data services and is in line with my duties - so think things like forms for degraded working and route maps. Its perfectly acceptable for me to plonk an ipad or phone (in flight mode) on the desk if necessary.
A prime example yesterday where I was cautioned over a small farm crossing that was given with mileage. Out comes the iPad to load up the maps and make sure that I correctly identify the location.
Some operators have a zero tolerance policy (like my former TOC), but this prevents many of the benefits that technology offers being realised.