Reference 1: PTT table for ring signal

 +--------------------------+------------+----------------------------------+
 |Country       V (Code)idx | Ring Freq. |  Ring Cadence (100ms)            |
 |                          |    (Hz)    |  On   Off  On  Off  On  Off  Ref.|
 +--------------------------+------------+----------------------------------+
 | USA          * (255) 1   |     40     |  10 , 30                     rc0 |
 | S. Africa    * (254) 2   |     40     |  10 , 30                     rc0 |
 | Netherlands    (253) 3   |     25     |  10 , 40                     rc0 |
 | Denmark      * (252) 4   |     25     |  10 , 70                     rc0 |
 | Switzerland  * (251) 5   |     25     |  10 , 40                     rc0 |
 | Sweden       * (250) 6   |     40     |  10 , 50                     rc0 |
 | UK             (249) 7   |     40     |  4  , 2  , 4 , 20            rc2 |
 | Belgium        (248) 8   |     25     |  10 , 30                     rc0 |
 | Greece         (247) 9   |     50     |  10 , 40                     rc0 |
 | Czechia        (246) 10  |     40     |  20 , 30                     rc0 |
 | Norway         (245) 11  |     25     |  20 , 30                     rc0 |
 | Australia      (244) 12  |     40     |  4  , 2  , 4 , 20            rc2 |
 | New Zealand    (243) 13  |     40     |  20 , 30                     rc0 |
 | Hong Kong      (242) 14  |     40     |  20 , 30                     rc0 |
 | Singapore      (241) 15  |     40     |  4  , 2  , 4 , 20            rc2 |
 | Finland      * (240) 16  |     25     |  10 , 40                     rc0 |
 | Marocco        (239) 17  |     40     |  20 , 30                     rc0 |
 | Taiwan (rack)* (238) 18  |     40     |  10 , 30                     rc0 |
 | Germany      * (237) 19  |     40     |  10 , 40                     rc0 |
 | Italy          (236) 20  |     20     |  20 , 30                     rc0 |
 | Ireland        (235) 21  |     40     |  4  , 2  , 4 , 20            rc2 |
 | Japan        * (234) 22  |     40     |  10 , 20                     rc0 |
 | Austria        (233) 23  |     50     |  20 , 30                     rc0 |
 | Malaysia       (232) 24  |     40     |  20 , 30                     rc0 |
 | Poland         (231) 25  |     40     |  20 , 30                     rc0 |
 | Russia       * (230) 26  |     40     |  10 , 30                     rc0 |
 | Hungary        (229) 27  |     40     |  20 , 30                     rc0 |
 | Slovakia       (228) 28  |     40     |  20 , 30                     rc0 |
 | Thailand       (227) 29  |     40     |  20 , 30                     rc0 |
 | Isreal         (226) 30  |     40     |  20 , 30                     rc0 |
 | Switzerland  * (225) 31  |     40     |  10 , 40                     rc0 |
 | UAE            (224) 32  |     40     |  20 , 30                     rc0 |
 | ChinaT         (223) 33  |     40     |  20 , 30                     rc0 |
 | China        * (222) 34  |     40     |  10 , 30                     rc0 |
 | Ukraine        (221) 35  |     40     |  20 , 30                     rc0 |
 | Portugal       (220) 36  |     20     |  20 , 30                     rc0 |
 | France         (219) 37  |     50     |  10 , 30                     rc0 |
 | Korea  mu-Law  (218) 38  |     40     |  10 , 30                     rc0 |
 | Korea  A-Law   (217) 39  |     40     |  10 , 30                     rc0 |
 | Spain          (216) 40  |     ??     |  ?? , ??                     rc0 |
 | India          (214) ??  |     ??     |  ?? , ??                     ??? |
 +--------------------------+------------+----------------------------------+

 For TA128, the optional frequencies are:               20, 25, 40, 50 Hz
 For E2864I and omni.net, the optional frequencies are: 20, 25, 30, 50 Hz