Power Outage Question

My U-verse voice and Internet system survived a 13-hour power outage very well. The backup battery unit kept my Gateway lit during the entire outage. My two wireless phones were out of commission during the outage. But I was surprised that my hardwired phone was operational during the outage.  Since my three phone, now with U-verse, operate through the Internet, I thought all three phone would be down. Is my hardwired phone still considered a landline phone and is thus immune to power outages? Or was my hardwired phone operational because the backup battery unit kept the Gateway lit? Any info would be appreciated.