The Hush-a-Phone decision applied only to mechanical collections, but the Carterfone decision of

For many years, the Bell System (AT&T) maintained a monopoly on the use of its phone lines, and what devices could be connected to its lines. However, the seminal Hush-a-Phone v. FCC case of 1956 concluded that it was within the FCC's jurisdiction to regulate the operation of the System. Subsequently, the FCC examiner found that as long as the device was not electrically attached it would not threaten to degenerate the system. This led to a number of devices that mechanically connected to the phone, through a standard handset. Since most handsets were supplied from Western Electric, it was relatively easy to build such an acoustic coupler, and this style of connection was used for many devices like answering machines.
Acoustically coupled Bell 103A-compatible 300 bit/s modems became common during the 1970s, with well-known models including the Novation CAT and the Anderson-Jacobson, the later spun off from an in-house project at Stanford Research Institute (now SRI International). An even lower-cost option was the Pennywhistle modem, designed to be built using parts found at electronics scrap and surplus stores.
In December 1972, Vadic introduced the VA3400, which was notable because it provided full duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit and receive. In November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar in design to Vadic's model, but used the lower frequency set for transmission. One could also use the 212A with a 103A modem at 300 bit/s. According to Vadic, the change in frequency assignments made the 212 intentionally incompatible with acoustic coupling, thereby locking out many potential modem manufacturers. In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and 103A operation.
Carterfone and direct connection[edit source | editbeta]
The Hush-a-Phone decision applied only to mechanical collections, but the Carterfone decision of 1968 led to the FCC introducing a rule setting stringent AT&T-designed tests for electronically coupling a device to the phone lines. AT&T's tests were complex, making electronically coupled modems expensive,[citation needed] so acoustically coupled modems remained common into the early 1980s.
However, the rapidly falling prices of electronics in the late 1970s led to an increasing number of direct-connect models around 1980. In spite of being directly connected, these modems were generally operated like their earlier acoustic versions - dialling and other phone-control operations were completed by hand, using an attached handset. A small number of modems added the ability to automatically answer incoming calls, or automatically place an outgoing call to a single number, but even these limited features were relatively rare or limited to special models in a lineup. When more flexible solutions were needed, 3rd party "diallers" were used to automate calling, normally using a separate serial port.