God is not limited to work in one way or another. He can work, and often does work, by a variety of methods. Here, in Acts 12, God sends an angel to open the doors of the jail. Later, in Acts 16, God will release Paul and Silas from the Philippian jail. But instead of sending an angel He will send an earthquake to shake the walls down.
Our God is the God of all Creation, don’t ever try to put Him in a box or think that there is some kind of formula or program that He must always work through. Be open to what God wants to do. If He wants to move in a new and different way, praise Him! Let us be open to the moving of the Holy Spirit and sing “Have Thine Own Way Lord”.