30 minutes is quite a long phone call, maybe they'd been sitting waiting for you, might have said 'we'll wait 10 more minutes and then head off'. They might have thought you didn't want to go out with them because you stayed on the phone for so long when you should have been going to the party. Simple solution, just apologise for keeping them waiting, let them know why you were sol long on the phone and tell them you tried to catch them up but couldn't get in. They'll know you hadn't meant to miss the party and then all will be good for next time. Best to be open about these things otherwise the misunderstandings just get bigger and more awkward.