FnF_VE
Going to a store should be your last resort. Use My T-Mobile/My T-Home (on their website). If you have a really special problem, call them. Also, never buy anything in a store or on the phone, unless you're very, very good at negotiating, you get 10% off of your orders automatically on the website. Customer service agents use the very same interface (with more privileges of course) but you can do pretty much anything you would want on My T-Mobile. E.g. I know for a fact (I'm also a customer) that you can disable receiving premium texts and having an Internet connection altogether. It's worth checking out every service if they're enabled or disabled at least once, so you can avoid unexpected charges and activate things that you know you might need some day. Btw, your options are very clearly described, much better than how an agent would explain them to you.
If you absolutely must see someone in person, here are some more tips. Always go to T-Pont stores, those are the official reps of the company, never go to a store with a big T-Mobile sign, those are resellers, agents there receive poor training and all they want is to sell you stuff. Don't go to a store in the late afternoon/evening hours, they have the most customers then and you have to wait more. 30 min waiting time is actually not very common, if you go any other time you either won't have to wait or only have to wait a few minutes. Also check on My T-Mobile if you're a 'gold' or 'platinum' client. If you are, always choose that on the first screen in the in-store queue ticketing system, and you'll get to someone in a few minutes even if there are loads of people in the store, bc most people don't know about this.
Yeah, it's ridiculous that you have to know all these to get a good service, but you don't have to put up with it, e.g. Telenor's customer service is pretty good, or so I've heard.