On the OBi Dashboard select your OBi110. Go into OBi Expert Configuration, enter OBi Expert. Under Physical Interfaces find Phone. There you can select PSTN as PrimaryLine. You probably will want to do the same for Auto Attendant. Press Submit at the bottom of each page and wait for the OBi to reboot before making further changes.
As you live in Spain, you may wish to consider making changes to various digit maps etc. The OBi defaults are set for North America. For example, you may wish to change 911 to 112 for emergency calls. 112 will work with default digit maps, but there will be a 10 second delay in sending to line as it relies on the xx. rule.
Digit Maps may be the subject for a future post
Good luck.