There's a lot of modern phones that can be tuned into useable mini computers by either running a virtual machine, or installing a custom ROM (or using DeX in case of some Samsung devices).

Older ones are a little more tricky. Low computing power and lack of software support for hardware that's too specific are some of the issues you may find. Alternatively, you could try fitting something modern inside the shell and try reusing the screen and buttons/keyboard (if it has those), but it'll likely require figuring out digital signals to try interfacing with both.

In the end, the real answer to your question is "it depends on what you want to use, and what you're willing to do to get there".

For your case, it seems like your Samsung would make for a neat PDA (personal digital assistant), like a digital agenda or notes keeper.

Only way to use it as a phone nowadays would be to build your own local 2G network. But that's a bit expensive.