Afaict old PABX's could work with either ordinary phones or analog phones with special features with most systems having a mixture.
Later the analog phones with special features were replaced by digital phones (using an ISDN like system I belive). I think most of them could still be configured to support analog kit though so people could connect thier fax machines etc. The phone system we have at uni certainly seems to have a mixture of ordinary analog stuff and digital stuff connectioned.
Some places have now moved over to VOIP systems. Even with voip systems though you can usually attatch analog equipment somehow if you need to.
Later the analog phones with special features were replaced by digital phones (using an ISDN like system I belive). I think most of them could still be configured to support analog kit though so people could connect thier fax machines etc. The phone system we have at uni certainly seems to have a mixture of ordinary analog stuff and digital stuff connectioned.
Some places have now moved over to VOIP systems. Even with voip systems though you can usually attatch analog equipment somehow if you need to.