Prepaid card users in some countries have this problem when using public phones to make the call. After the user of the prepaid card dialed the access number and the greeting prompt is heard, the public phone does not respond to any digit input anymore and the phone simply goes silent. This is a deliberate and anti-competitive act by the big telcos to prevent competition. Somehow these telcos’ own prepaid card is not affected by this. How can I overcome the problem?
I do not think that it is specially made to mess with VoIP operators, it is to close some security holes in the payphones, which phreakers used to use in the past to place free phone calls by emulating tones (“coin collect”, “coin return” and etc).
Read on Internet about “Green box” and “Red box”. I think that these two are the ones related to the payphones.