$myvar is wrapped in single quotes, $mnumber is not. It doesn't matter that the innermost quotes are double, it's the outermost quotes that the shell uses to determine if it should do variable expansion or not.
Rewrite it like this (I shortened the URLs to make it readable):
curl -Ld'...dmobile='$mnumber'&message="'$myvar'"' http://...
Notice the extra single quotes before and after $myvar.
