It is true - in many Mormon homes. Unnecessary, external influences i.e. TV, phones, internet, are controlled and/or avoided whenever practical.
I would think a phone would be an added safety measure in a house as large as the Smart's. No one in the know has mentioned it one way or another, but MK could have called the police if there had been a phone.
A 6000 sq. ft. home is large, but not huge.
Apparently that didn't apply to computers. Seemed to be plenty of those among Smart family members. The total number LE took was 13, as I recall.