I don’t see the “accent” option on my iPhone keyboard either.
Press the small case ‘a’ on the iPhone keyboard and hold it. Above the ‘a’ you’ll see a series of a’s with various accents. Slide your finger over the one you want. On my keyboard, the à is the second choice from the left. Voilà !
If you copy this
voilÃ
And paste it into a document, run spell check and add it to your dictionary, it should be available to you, all the time, thru spell-check.