"You have a really small cell phone."
Translation:Sinulla on todella pieni kännykkä.
The younger you are, the more likely you are to use just "kännykkä". Kännykkä means a cellphone, but just a few years ago people tended to mean the landline when they talked about puhelin. Nowadays you can use puhelin when referring to cellphones, too, but it's not as common as using kännykkä.
Correct. The answer "Sinulla on todella pieni matkapuhelin" should be accepted.
Kännykkä used to be slang, now commonly accepted as part of the Finnish language. "Känny" is often used in spoken language as it's even shorter.
"Puhelin" would still most often be understood as telephone in general. "Puhelinnumero" is "telephone number", for example, and "puhelinliittymä" refers to your (cell/landline) phone contract with your operator.
"Lankapuhelin" would be what is known as the landline (phone).
In many instances it's understood that "puhelin" will refer to a cellphone. For instance:
"Mikä puhelin/kännykkä sinulla on?" (What (cell)phone do you have? "Miksi et vastaa puhelimeesi/kännykkääsi?" (Why aren't you answering your phone?)