Doesn't Sonic offer a DSL + phone line plan? I found these plans on the sonic.com/availability page, after entering my address.
I've had Sonic as my ISP for years, first as a dialup account, and then DSL. In both cases, I understood that Sonic uses ATT's phone lines to make the connection physically. So if ATT now uses fiber for their phone lines, wouldn't the connection I already have be over fiber?
If you are a current Sonic customer, instead of looking up your address on the main website, I believe it is better to check the Member Tools page