If you're moving to Turkey, Your house or apartment will have a three-prong wall receiptical that takes a three-prong plug with the prongs in a trangular configuration, not an RJ-11 plug. However, the "modem" end of the telephone line has an RJ-11 style plug on the other end which can go into your modem or telephone. If someone physically comes to install your telephone, they will set it up that way. So if you'll be working with an existing telephone line plugged into the wall, you will not need any type of adaptor at all.
If there is no existing telephone line plugged into the wall, you can get adaptors which have the three prongs and a receptical for a US style telephone as well. Or you can get the kind that you have to unscrew, then wrap the raw telephone wires around screws and tighten them, reassemble, and plug into the wall.
I have also simply removed the covering and spliced a US style telephone line directly to the wires and everything worked fine. No adaptor necessary, just a steak knife!
If you're visiting Turkey and staying in a hotel, the telephone lines are generally the same way, that is, they will plug directly into your modem, and the better hotels have telephones with dataports so the phone will still ring while you're online. I have seen hotel phones with wires that cannont be disconnected, but I think only one or two times, and these were old phones they probably got on the cheap. If you stay at a five star hotel you shouldn't have any problems at all. Some even have WiFi or an Internet service for a fee at the hotel.
You didn't ask about this, but it may be worth mentioning. The electrical plugs are different from the US plugs. In Turkey they use a standard European style electrical plug with 240 volts. While the prongs on the US plugs are flat, the European style electical plugs have two round prongs. You will need an adaptor for that, which may be difficult to find in Turkey depending on where you are. Your best bet is to see if you can find a European electrical adaptor online or go to a do it yourself store like Pratiker in Turkey or to one of the larger department stores, since there is not much demand for these kinds of things in smaller towns. I'm operating on the assumption that your computer will take 110 and 220 volts... You might make sure it is dual voltage before you plug it in to a 220 volt receptical!
Hope that helps. Where are you visiting in Turkey? Are you moving to Turkey or just visiting?
And by the way, welcome to the forum, we're glad to have you!
Ken