If you are a Christian, everything you do should be done with reverence toward God. Therefore, as He has given all of us the call to make disciples, (Matt. 28:19) we all must be involved in missions. Some to go; some to send; but all to speak and tell others.
You may feel you are not called to go overseas, and that's OK. If an army were to send all its men and women to the front lines they would lose the battle. Because there would be no one to care for the wounded, no one to replenish the supplies and no one to prepare food for the soldiers.
If we have the calling to go to Africa, that is highly dependent upon many people not going. It is dependent upon those people being called to serve as senders.
If all Christians were to go into full time missions - to the "front lines" if you want to call it that - we would soon have to come home. There would be no one to replenish our supplies, to give us the prayer and financial support we need. We need people who don't go. We need people to serve as senders.
But those also should be telling others about Christ.
Jesus said, "You will be My witnesses both in Jerusalem, and in Judea and Samaria, and even to the remotest parts of the Earth." (Acts 1:8) We can't all be in all places, but we can all be witnesses. Some are called to remote parts of the Earth - Like Lesotho. Some are called to witness to a region, like a pastor. All of us are called to be witnesses to those around us day to day.