It shouldn't let you delete system apps, unless the phone has been rooted that is.
The only way to actually get them back, would be a "Hard Reset" of the phone. This would get the phone back to its original state, however, that would also remove any and all personal data from the phone.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up...