Before you flash a modem, try another dialer app, just in case it is at fault. Check if 3g/LTE/WiFi are working at all. Chances are that it is the modem that is causing the issue but it is easier to install an app or two before flashing basebands.
On the Samsung Galaxy S4 International LTE model: GT-I9505 you cannot flash a Modem through recovery. You must use
Odin to do this.
Complete list of Modems for GT-I9505 and I9500 is here.
It is with noting that sometimes the first flash fails, and can even read as "Success" in Odin. Untick reboot and try it twice, then reboot the device.
You man need to try a couple of modems, but try initially to flash the one relative to your ROM (in your case
XXUFNAD). However, the baseband (modem) can be different depending on your region/carrier/settings/ROM so a little trial and error is required. You may be using a 4.2 modem for example and this probably won't work. Most 4.3 and 4.4 modems will work fine in KitKat.
Edit: This is the recommended modem with the NEW bootloader. This modem is
XXUEMJ5 version. Try this first. Then experiment with others.
Flash it in Odin as below, HOWEVER, do it separetely without the bootloader option.