i was about go to sleep my mother telephoned me,这里的about什么意思

原句应该是：
I was about to go to sleep when(这时：并列连词,不能省的）my mother telephoned me.
be about to +v马上要,就要（不能跟时间状语连用）
我刚要睡着,这时我妈给我打电话了.