Only about 5% of babies are born on their due date, though most of them arrive somewhere between 37 and 42 weeks. From about 37 weeks, therefore, you'll need to know how to get in touch with your birth partner, in case your labour starts and they're not with you.
Since the advent of mobile phones, keeping in touch is now much less of a problem. If you don't have a mobile though, there are companies that will rent out phones or pagers specifically for this purpose. Look in the classified ads at the back of pregnancy magazines for details.
If not being able to contact your birth partner easily is something that is worrying you, ask them to give you a schedule of where they'll be each day, and how you can get hold of them.
However, if you should go into labour and can't immediately get in touch with your birth partner, don't panic. In all probability, you'll still have plenty of time to get hold of them. First labours usually last more than six hours.