Can an aupair use the host family's phone?

Well usually the host family will allow you to use the telephone once when you first arrive to inform your family that you arrived safetly

Any other time you need to ask the family if you can use the phone.

Alternatively if the family have allowed you internet access why not ask their permission to download free software and this allows you to speak to your family ( or anyone else who has the software ) free of charge. This so software can be downloaded from: