Is there a phone at every level crossing?
I describe myself as a rail enthusiast and I don't know the answer to this question, so I don't know why you would expect the average member of public to know that (a) there is a phone, (b) that it's there for public use in appropriate circumstances, (c) that this is an appropriate circumstance, and (d) that this will directly contact the signaller (not a call centre or something).
Also, are you sure that even if they did know all that that, stopping, going back to the crossing, finding the telephone, working out how to use it*, waiting for the signaller to answer and explaining to them signaller is going to get a quicker response than calling the police from your bluetooth headset immediately?
*I've never had cause to use a level crossing telephone, but I remember reading an accident report (one of the early RAIB ones I think) where someone failed to get in contact with the signaller because they assumed the phone worked like the motorway emergency phones they were familiar with. The motorway phones connect automatically when you lift the receiver, the railway ones - at least at the crossing concerned, you have to press a button - but all the illumination of the instructions telling you this went out when you lifted the receiver (the theory being that this highlighted the button needing to be pressed). I have had to use the motorway phones on I think three occasions, and had I not read the report in question I strongly suspect I would have assumed they worked in the same way.
No, there isn't a phone at every level crossing. However, there will usually be a way to directly contact the signaller, whether that's a level crossing phone or an adjacent signalbox, for example. (Foot crossings are different, but unlikely to be relevant in this specific scenario.) AHBs are usually in areas remote from signalboxes - in fact, I can't think of any which are immediately next to a box - and therefore tend to have a phone contained in one of the distinctive yellow boxes. This is for the use of the general public and train crew, if need be, and can sometimes be used by road vehicle drivers for non-emergency matters. At various crossings - not AHBs, but usually "occupational" crossings with manual gates - the phone will sometimes be used to confirm the line is clear for any vehicle to cross. At others, including AHBs, signs will either direct the user to use the level crossing phone or to dial a specific phone number for permission to cross with animals or cumbersome vehicles, which might take longer to cross than the automatic warning sequence allows.
There are also various different types of phone around the network which contact the signaller in different ways. Some may have one or more buttons for different lines, others (like SPTs) may connect automatically once you lift the handset. Away from level crossings, there are other types of phones, including Emergency Telephones, which feature a keypad for dialling within the private railway phone network.
When it comes to level crossings, I should imagine there are at least one or two types of crossings which each individual road vehicle driver will have never seen. This will vary around the country. In my area, most drivers will never have seen an occupational crossing with manual gates or even manual barriers, but 20 miles away, they might have several in their area, but no AHB crossings. There seems to be little emphasis for learner drivers in being assessed on all the different types. Even train crew may not readily remember the precise nuances of every single variation, even if they know the crossings in their patch by name, location, whether or not it's monitored, and an approximate level of risk. Outside technical Network Rail circles or this forum, I also doubt there are many people who know what each acronym means!
And in terms of emergency phones, it's hard to know what people think they should be used for. So much emphasis is placed on getting people away from crossings if there's an incident and a train approaches - but relatively little on what the appropriate contact methods are.