Although ironically that isn't there job. I'd imagine the number reported and actually actioned was a very small percentage of the total number of vehicles on the road. And would likely require the expense of people to follow up on such reports, of which I'd guess a lot were false or just someone trying to cause trouble.
The DVLA and other authorities have plenty of ways to check vehicles, they really don't need vigilante style people who don't work for them.
As for phone signal. Well I've been to Wales twice this year on Land Rover adventures. And in the middle of know where I've often had way better signal than I get at home, 50 miles from London.
Probably also worth mentioning access to colour photocopiers or printers is a lot easier these days. Printing a 'fake' tax disc would be no bother to fool the wannabe do gooders.