Well, that's really two questions, but the first is rather easy to answer, the second will take some time. No, we don't have to call God 'Father.' Throughout the Bible God is addressed by a wide variety of names including Rock, El Shaddai, God, Lord, and so on. Certainly, Father is a valid name and image for God, but it isn't always the most appropriate, especially in public worship. Current statistics indicate that nationally 1 in 4 women have been abused by a male figure, and many, many women (and men) have been abused by their father. Although the image of God as Heavenly Father is meant to indicate the ideal father-figure, for some, any reference to "father" brings an involuntary cringe deep inside and erupts feelings of revulsion. Certainly it is incumbent upon the church to be aware, and indeed to be wary, of actions or words that might keep any child of God from entering into the fullest possible relationship with God. So no, we don't have to call God father, and it seems to me worship leaders and prayer leaders should refrain from that particular image in public worship.
However, for several millennia the people of faith have been referring to God primarily as a member of the male gender. Most images of God in scripture and in tradition use a male model: Father, Lord, King, He, Him, etc. Recently, however, many of the church have been examining new images for God and the female side of God has been thrust forward as one of these.
However, God as Mother is not a new image. It is as old as the Old Testament. Indeed, from the creation story we can begin to see another image of God. In Genesis 1:27 God creates the humans in the image of God, "male and female God created them." Thus, in some way the female here is also in the image of God, or God is in some way of the image of the female. But the image of God as Mother doesn't just end here. God is imaged as having a womb: Isaiah 46:3 "Hearken unto me, O house of Jacob, and all the remnant of the house of Israel, which are borne by me from the belly, which are carried from the womb" (KJV), and in labor in Isaiah 42:14, "For a long time I have held my peace, I have kept still and restrained myself; now I will cry out like a woman in labor, I will gasp and pant" (NRSV). And lest one think only Isaiah has images of God as a Mother, note that in Hosea 13:8 God is pictured as a mother bear protecting her cubs and in Luke 13:34 as a hen gathering chicks, both maternal acts.
But isn't God a male? No. God is neither male nor female but spirit. It is our lot in life to use metaphors to try and grasp the unfathomable, ungraspable divine God, so we use what we know to try and understand the unknowable. Male, female, rock, she-bear, hen, Lord, anchor, creator, and on and on, all different metaphors to describe God. So can we call God 'Mother'? Yes, and perhaps we should call God 'Mother' sometimes, just so we can remind the church and each other that even the Bible uses this picture to see God.