It actually was a ground based system using low frequency and a surprisingly small number of base stations. Some widebodies had satellite equipped phones but all domestic stuff was routed to the ground. Interestingly, this ground based network is the same system that GoGo is running on.
Im pretty sure the RJ-11 was for a dial up modem.
When Verizon acquired GTE they’ got creative and enabled calling for Verizon customers for something incredibly cheap like 10 cents a minute.