Why telephones are made of plastic?

Most likely polypropylene (PP) For it's harder qualities but it could be Polystyrene (PS)

Just look for the recycle symbol on it. Whatever number is there is the type of plastic related to it.

1. PET (PETE), polyethylene terephthalate
2. HDPE, high-density polyethylene
3. PVC, polyvinyl chloride
4. LDPE, low-density polyethylene,
5. PP, polypropylene
6. PS, polystyrene