The difference in the example you gave is pretty subtle and they are almost interchangeable.

"I meant to call you" implies that you were going to call someone but then forgot (or got busy). You might say it if you run into someone that you had intended to call earlier that day or in the last few days.

"I wanted to call you" could mean the same as "meant" or it can also imply that some other reason that it out of your control. e.g. the power was out.

Canadian English.