It is saying the period of time before your Obi should "register" again with Phone Power is 240 seconds. Your setting is probably less than that but it isn't hurting anything. Mine is currently set for 180 seconds. Setting the registration interval higher uses less data.
This shouldn't have anything to do with your relative not being able to call you. Log into the PP web portal and look at your Call log/usage. Do you see the phone call from your relative listed there?
Go back to the SIP Credentials page under Modify Services. What do you see under Registered User Agents?
When you go into the Obitalk Dashboard does it say "registered" under Status? I can tell you how to set your registration interval to 240 seconds but it would mean going into Expert Configuration. I realize it is intimidating to go into Expert Configuration. But just about everything is intimidating until you've done it a few times. Right?
You can't do any potential damage unless you uncheck the ObiTALK settings box, change the setting, and then click on the Submit button at the bottom of the page. And even if you did mess something up you now know how to start all over......which should take you less than 5 minutes. I would recommend you become comfortable with the procedure of starting over again and reconfiguring everything. Do it for drill. It doesn't hurt anything and you will then become comfortable enough to start fine tuning your configuration (like registration interval). If something gets screwed up....big deal. Just start over again and you'll be back up and running in 5 minutes. Understand that your subscription with Phone Power is heavily discounted and a BYOD (bring your own device) service which means you are responsible for the correct functioning of your OBi, not Phone Power. Tech support may try and help you but you can't count on them as you apparently found out.
Just my opinion. Others may disagree.