Some people have had some issues with their Samsung s²² series of phone's which has mainly been tackled by software updates sent out by Samsung @PaulC59
My Samsung Galaxy s²² Ultra has been fine.
Some generic troubleshooting tips are .....
- Clear Cache Files from the phone's System Partition Section > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too.
- Safe Mode > https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/ turns off downloaded apps and themes etc which may highlight an app or theme that's causing conflict.
- Factory Reset after backing up and as a very last resort.
- You can send feedback and any relevant error reports via your Samsung Members App.
In regards to calls dropping > Does this happen everywhere or in a particular area ?
As a process of elimination can you try your sim card in another phone.
And if possible another sim card in your phone.
In Settings > General Management > Reset > Reset Network Settings.
In regards to Wi-Fi dropping > Does this happen to any Wi-Fi source you connect to ?
If it's your Home Broadband Wi-Fi then try forgetting the network and reset the connection back up.
Some have found it beneficial to change wireless channel in the router.
Also some have found splitting the routers bands to 2.4ghz and 5ghz and then connecting the phone to one and then the other has helped as the phone may prefer one over the other.
A Samsung Service Centre can check out the phone.
Your phone also has a diagnostic test tool in your Samsung Members App.
I wish you all the best. 
