Continuous DTMF generation is one of the essential functional properties of any type of phone to avoid extra call setting charges. Other functional features of analog phones like IVR (Interactive Voice Response) operation, and remote access to TAD (Telephone Answering Device) systems, also require generation of continuous DTMF.
Some recently evaluated devices are not capable of continuous DTMF generation. These products typically limit DTMF generation to bursts of 100 ms each time a key is pressed. This condition impacts name branded calling card customers by not allowing them to generate a DTMF star for the required two seconds, in order to continue placing calls without re-entering the card numbers and PINís. As a result, these customers must pay a call setup charge for each call, instead of one setup charge for a group of calls. Some other networks are designed similarly, potentially impacting all users of these products. Likewise, some IVR (Interactive Voice Response) systems fail to recognize a user's attempt to provide a long DTMF entry, as is needed for proper control of the system by the user.
It has also been noted that the remote access and operation of a TAD (Telephone Answering Device) is a problem from an analog phone which can not generate DTMF continuously. This problem becomes serious when the user at the remote point wants to delete all the stored messages to make room for more incoming messages. To accomplish this, the user must press and hold a particular key until a confirmation tone is received to indicate that all the messages have been deleted.