Renew it. I wouldn't consider site traffic for a .tel domain as a factor for keeping it or not. Would you get rid of your phone number if you were only getting a few calls a day? I would consider site traffic an important factor for any other domain, but .tel is a contact page, and you don't want thousands of hits unless you want thousands of phone calls, texts and emails.
I think it's a great idea for a single digital identity, especially if you can get a short, memorable name, but the people running it are doing a lousy job of promoting it. It doesn't replace business cards, but it makes them much cleaner. I had my cards printed simply with my name and .tel address, and almost everyone who receives the card has never heard of .tel, but they're impressed with how easy it is to access me on their smartphones.
I don't know what will happen with .tel with all the new TLDs that will be introduced in the next year. I hope it doesn't get lost in the shuffle. They had a four year head start and didn't make the best of it.
You might want to consider switching to different registrar. €18 seems a bit steep. I pay less than half of that!