Here is an article on doing the necessary port forwarding, for your router.
http://portforward.com/english/routers/port_forwarding/Dlink/DSL-3780/3CX.htm
If you do some searches, you'll come across a few sites like this...
http://lifehacker.com/the-best-free-alternatives-to-dyndns-1561556205
As your router doesn't seem to have the ability to be set to use a DDNS server, you will have to run one on any computer on your network, including the one running 3CX.
Before you really get into this, are you planning on using "remote" extensions? That is the most common driver behind having a Static IP or using DDNS.
Have you checked as to how often your public IP is changing? Sites such as http://www.displaymyip.com/ will tell you what your public IP is, perhaps it doesn't change that often to be bothered going though all of this.
Some network devices, that you may have installed, Web cameras, file servers, and the like, will come with a free DDNS service that you can make use of.
DDNS is very easy to understand, it's like directory assistance when your phone number keeps changing. People don't need to know your number, just your name.
You go to a site and sign up. You choose a 3 part name such as bobspbx.mydynamicip.info, you usually only get to type in the first part, and have a limited selection of the last two to choose from depending on large the outfit is that you are dealing with.
You will also choose, or be assigned a user name, and a password. They will give you their server address as well.
The information that they supply goes into, either a router, or a stand-alone programme.
The router, or programme, will "report" to the server name, log in with your user and password, and update your public IP against the name
Most routers are capable of knowing when your IP changes, the programme may not, and may have to "check-in" on a scheduled basis. When you do a DNS look-up on your "name" (bobspbx.mydynamicip.info), your current IP should be returned, and that is what will be used to route to your router.