Dialup modems are affected by a number of variables including the length of the phone line. It is important that the modem be configured correctly in order to establish a reliable and quick connection. Faster is not always better either, the typical setup time for a V.90 56K modem is 40 seconds, the setup time for a V.21 300 bits/second modem is 1 second and usually a whole lot more reliable. 2400 bits/sec usually works well too.
You will need to consult the instruction manual for the modem and experiment with different init strings in order to find a setting that will work reliably all of the time.
(The mobile number 0499999999 is used as an example here.)
TAP Protocol example (Two SMS sent):
Protocol Example - Bad Checksum
Protocol Example - bad destination (in this case: destination number missing) You will receive the same response if the line has Send SM barring or Silent line with no override