Since these are all string literals you really don't need any concatenation function at all except to concatenate with the output of getIMEI().

  char *a = "Hello " "world!";

Works just fine.

Edit to add: You can really see the difference in code between someone coming to C/C++ from a high level language and someone who learned assembly first, where a list of literals is a common idiom. The original style is not functionally wrong, but it does look like Java :-)

Also: DON'T post your potentially insecure string handling code on the Internet; are you crazy?