So you can dial out, over your providers trunks, but only to landlines? If that is the case then it may be one of two things...your outbound rules are not set-up to include mobile numbers, or, your provider blocks calls to mobiles because of additional cost, perhaps an option that you have not set up. The 3CX logs will show you how far the call gets. If it is being sent to the trunk group, and the number format looks to be correct (you may need to confirm that), then you may have to speak with your provider to find out what is going on.