Matthew 28 is sufficient authority; lack of a calling is not a reason or an excuse
“I went to _x_ because I was called by God.” In this kind of conversation, the calling was an additional push or reason to go.
“Why am I not in missions? Because I’m not called.” This is really a false line of reasoning.
Matthew 28 is a command given to all of us to obey. It’s all the authority any believer needs to go, teach to obey, and baptize.
Now, there are certain conditions under which most agencies and churches will say a believer needs to not go or delay going–if a believer is locked in a pattern of sin, for example, or in most cases if a believer is enduring a family crisis or a debilitating disease, or whatever.
But not for one minute should we think that, just because we have not heard an audible voice or seen a burning bush, we cannot go, witness, make disciples, baptize or teach to obey.
These commands are binding on every believer, young or old, Jew or Gentile, male or female, rich or poor, slave or free, sick or well, legal or illegal, new in the faith or old in the faith, married or single, childless or parent, whatever.
If you want to go to the nations, and you’re willing to walk a path of obedience, learning, accountability and bravery, you can. It’s as simple as that.