This question is quite the quagmire... or is it?
Clearly, Ishmael was the illegitimate son of Abraham. And Abraham was God's man who had his faith "reckoned" to him as righteousness (or had his faith seen by God as what was needed for him to be considered forgiven of his sins). Ishmael was saved in the desert with his mother, Hagar, by an angel of The One True God. Hagar had prayed for help and this prayer was answered by Jehovah, with Him dispatching His angel. All this to say, that although Hagar and Ishmael may have known and experienced the God of Abraham, does not mean those alleged descendants of Ishmael who created the religion of islam were inspired by the One True God. In fact, one can clearly argue against the premise that allah is the God of Abraham based on the writings of the qur'an being compared with the Holy Scriptures.
The argument that Ishmael, the son of Abraham, must by birth and tradition have given rise to descendants who created a religion who also worshipped the one true God is logic that is fatally flawed. Although I have known Christian missionaries who assert to islamists that allah is the One True God of Abraham, we clearly see in the gur'an, allah has no son. The God of Abraham, Isaac and Jacob clearly has a Son, according to the Bible (see John 8:58). Thus, allah is NOT the God of Abraham...