This will largely depend on few factors and feature of the tab
- If you just bought it, then you may have to launch Google Play and download the Phone app from there and it will come with a dialer
- Your tab's Phone app cache may have gone corrupt requiring that you hard reset it
- The phone app files may have gone corrupt requiring that you should uninstall it and re-install it.
Kindly comment below with the tab's manufacturer name and model number and we will gladly assist further should the fore said be of no assistance due to being more generic.