You most likely have some sort of software conflict. Not all applications play well together. Have you added any new software recently? If so the new app is probably conflicting with something else. Try deleting it and see if that solves the problem. Most commonly an app that runs in the background will cause problems. The problem as you describe it sounds like a background app.
If the problem persists on the dial pad enter #*377 and press dial. This will give you information on the error including which app caused it. The app shown may be the actual cause or it may be conflicting with another app that runs in the background. Do this each time you get an error. If the error information shows different apps then it is definitely a background app problem.
If all else fails, hard reset and reinstall your apps one at a time and each time give all the apps a thorough workout over several hours. If the problem recurs that last installed app should be the culprit.
Good luck.