Magti is usually preffered operator with probably the best coverage. In Some unpopulated/less touristic places, instead of 4g coverage you may have just 2g anyway. In most cities there is at least 3g if not 4g. No coverage in some areas like Shuapkho or Tobavarchkhili lakes. For international calls beeline may have special tarriff plan so you can ask.
There are 3 main operators. You could check rates of internet or some other services: