Phone Home From Vietnam?

Did US service personal really have the opportunity to phone home from Forward Operating Bases during the Vietnam War, before the days of the mobile phone? It is good if there were facilities in place that enabled this to happen but I am surprised if this was the case.

The reason that I ask the question is that in a documentary on the Vietnam War that I have been following recently, an interview with a US veteran, seem to suggest that he could "phone home" from a F.O.B. in Vietnam. I could, of-course, have miss heard what the veteran said.

During the '70's we were allowed a phone call each week free through our own system when we were in operational areas... The US system was huge by comparison. Anyway, we have lots of RVN vets here so...

From everything I've read the only ones making phone calls were the ones on R&R or wounded in the hospitals or the rare ham radio like link up

I recall the MARS network.
I believe it was Ham operators.

Use of the MARS was for emergency and special occasions, not everyday communication. Death in the family or severe sickness, birth of a baby, wounded calling home. The calls had to be approved by Battalion commanders. It was like a radio call with an operator relaying when the person could speak, using the term "over" was used to tell the operator when the other person could speak.
There were MARS stations on all US Army posts. The sending overseas MARS station would contact the Conus MARS station closest to the soldiers home. The stateside MARS station would contact the phone number the soldier would give, then with the connection made the parties would talk to each other using the operator. Good communication depended on atmospherics.