Yes, phone calls will interrupt the session and currently the entire movie will be lost. Saving photos will keep all photos as they come in so you won't lose them. You are correct in stating that airplane mode will ensure the session doesn't end.
I'm currently working on some improvements to address unexpected interruptions such as phone calls. Depending on the exact circumstance it will allow TimeLapse to recover all or part of the movie generated until the interruption.
I hope to submit the improved app for Apple review in the very near future.
I appreciate your feedback and your patience while I get this resolved.