Acid-base reactions are generally the fastest reactions you will see. If it is possible for an acid-base reaction to occur, it will generally happen first, and if there is a large difference in pKa's it will be essentially complete. Grignard reagents are very strong bases and phenols are fair acids - the Grignard will be protonated very rapidly, and if you only use one equivalent, you won't see any addition.
If you use two equivalents of a Grignard, you can sometimes still get the reaction to occur even on the deprotonated compound - Grignard reagents are also good nucleophiles.