I'd suggest we need both as all phones are not necessarily smartphones. In fact all the disaster folks I work with use non-smartphones (such as the Nokia 6210i) specifically as they do not require the same sort of infrastructure as smartphones, so will still work in major disasters. Also, their battery life is weeks rather than days.
The security implications for each are different as well. A smartphone typically has a full tcp/ip stack, and a fully functional OS. 'Dumb' phones are very limited in functionality, so can be much more secure, and are certainly not susceptible to the same kind of attacks.
The phone tag is about the traditional telephone network. It is for issues with dumb phones or the aspects of your smart phone that makes it just that - a phone.
On the other hand, the smartphone and mobile tags are about the aspects of your phone that makes them tiny mobile computers. The distinction between the two seems to be that mobile includes tablets while smartphone doesn't. There is no specific tag for tablets. In my opinion these two tags could be merged.
I don't see the point with a cellular tag on top of the others. Since it has only three questions I think we could just kill it.
And all of these needs better usage guidance. This is how I would organize it:
mobile-devices (merge of mobile and smartphone):
Use for security issues specific to mobile devices such as smartphones and tablets.
Use for security aspects of phone networks, phone calls, SMS, etc. For issues with smartphones, use mobile-devices.
Improvements and other suggestions are welcome.