I'm trying to do this too, but don't want to lose my phone insurance. There are some data points on DOC that Verizon gift cards work, but the smallest increment is $25.
FWIW the charge was for ATT MOBILITY on my bill. Got the credit two days later. Our phone bill is separated into 4 different lines. And so I figure we are okay if one phone with a $55 line needs the claim. Since the $20 easily could come off something else. I’m prepaying the bill so its a credit on the months original bill.