A health trust is investigating after all the telephones at Whipps Cross Hospital were cut off for most of yesterday.
Staff contacted the Guardian concerned that they were having to use their own mobile phones to make work-related calls due to the apparent fault at the Leytonstone site.
But Barts Health NHS, the trust which runs the hospital, has insisted no patient care was affected and no operations were cancelled as a result.
A spokeswoman said the phone system was restored at around 4pm and an investigation was underway.
It is believe the phones stopped working first thing in the morning.