You are looking for a Gateway then, a "box" to connect 3CX to telephone lines from your local provider? If so, you have two ways to go, it all depends on how many lines you need now and plan on using in the future. There are single line units, which while less expensive, will add up it you will eventually need four or more lines. There are other devices that handle four, eight or more lines. Many have been discussed on the forums. Certain units are favoured by some users as being reliable and easy to set up. Other users will base the purchase on price, willing to trade off some features and ease for a more economical device.
If you narrow down what you feel will best serve your initial and future needs, people on the forum might be able to come up with some choices for you.