We typically use something analog from a vendor that manufactures devices for outdoor or other environments such as Gaitronics, Ramtel, Allentel, Talk-a-phone, or Ramtech or something. Not specifically in a freezer but outdoors in winter and such.
Most electronics don’t seem to be rated below 32F even though they may “work” , due to probably some sort of testing and rating requirement.
Getting into PoE heaters and things just seems to be pointless - save for “monitoring” though many of the analog phones have some ability to dial in and test if they are designed for call-out primarily.
