It is not uncommon for a Legacy PBX to need to co-exist, at least for a time, with a new IP PBX. Perhaps management wants to try-before-buying or see the new system working before dismantling their 20-year-old system. As working with Legacy equipment is a reality in everyday life, we will go over integrating 3CX with a Legacy phone system. We will use an ATA device that converts an analog device into a SIP extension, which connects our Legacy PBX to our 3CX Phone System.
At our office, we replaced our four-line Panasonic KX-TG4000B telephone system with 3CX, as shown in the following image. We'll use that as our example, but remember that the process is largely the same even with a much larger system.

We'll use this Legacy four-line Panasonic phone system as our example. As it is a four-line system, we can have up to four concurrent calls between 3CX and this system.
The basic steps are as follows: