Ubuntu Networking :: RPC Works For Some Things And Fails On Others

I'm trying to use a remote procedure call. When I call my server, my server should activate gammu and send an sms with it. I've used the code in the following tutorial: [URL]. The command for the uptime, and the command for the greeting work perfectly. But when I write my own method on my server, it fails ..

Code:
function uptime_func($method_name, $params, $app_data) { return `uptime`; } function greeting_func($method_name, $params, $app_data) { $name = $params[0]; return "Hello, $name. How are you today?"; }
function gammu_func($method_name, $params, $app_data) {
$text = $params[0];
$number = $params[1];
$result = "echo '$text' | gammu sendsms TEXT $number";
exec("$result");
return $result; }
On my Client (the one that calls the server) I see the output of $result. So my gammu_func is definitely working... He just doesn't execute
Code:
exec("$result")

I know that syntax is right cause I tried it in a different php file. I think it has something to do with the user rights. I don't think I have the privileges to run that command ...