Which country are you in and which service provider do you use for your Sim..? Please check here as well Kospet Optimus 2 Imei white/black list
It’s quite possible that your service provider may not support VoLTE for this watch. It depends on many factors to be honest. However, your situation is interesting because you can use it sometimes, 
There are certain bands that your service provider may use. A good start is to find out what the bands are that they use (Web search your Sim provider)
Next steps are to check out the watch bands in the engineering menu (search for MTK Engineer mode on this site for guidance) and make sure your watch is set to use them. Next, when you can connect, are you able to make phone calls and use data on LTE…. If it is data only that works, then it may well be the Sim provider that is preventing voice calls (VoLTE)
You can also research accessing the built in test tools on the watch and look for a modem test option. Or similar… and run the test.
That’s the first steps to trouble shoot this problem. This watch you have just purchased was made a long time ago and is “new old stock”. I really hope that it is not a hardware problem because it is almost impossible to get a replacement or a repair. Regardless of what the seller tells you in the advertising.