Hi Oscar,
For executing RFC and BAPI from external system the user should be a System(Background) user.
As you said you are able to login from dialog mode, seems like the user is a dialog user. You need a system user with RFC authorizations.
If the error still persists, then check the Z_FI_EDITRAN_NORMA_43, whether this is a remote enabled function module or not. To check goto SE37 Attributes tab-> check FM type. If normal then click on remote enabled radio button, Save and Activate it.
Few links that could help you.
http://scn.sap.com/message/4899551
https://scn.sap.com/thread/1031307
http://listserv.sap.com/pipermail/linux.general/2001-November/001820.html
http://help.sap.com/Business_Packages/EN/ca/115e4088dc0272e10000000a155106/content.htm
Regards,
Prashant