AT&T employees are working around the clock trying to restore phone, Internet, television and home alarm system services to about 300 Frisco customers.
The fire disabled the server that delivers the electronic signals controlling those systems -- including the ability to call 911 from a landline or home phone -- at about 3 p.m. Thursday.
A passerby noticed smoke and called authorities.
AT&T security is working with fire investigators and police on the incident.
Fire Chief Mack Borchardt said AT&T had recently been working on the box but said the the box had been sealed. He said it appears the fire started in a circuit board but called this kind of fire highly unusual.
Police said they will have extra patrols in the area of Teel Parkway and Fox Ridge Trail, the primary affected area.
AT&T said it anticipated that most customers would have service restored by Christmas or within the next 48 hours.