Hi Sina, you can check with (as root) rpm -q kernel-sourcewhether the kernel sources are installed. If not, install them with YAST. Search with "kernel" for available packages. And look in the result list for a package "kernel-source". This is the full source of the SuSE kernel including all there patches. Do NOT install the kernel-vanilla package, as this is the original kernel from kernel.org, which does not match your running kernel.
Then try to compile the modem driver again. Regards, Joern