Learn Spanish (Latin America): Necesito el teléfono.

In Spanish, the ending of the verb tells you who is doing the action. Necesito comes from the verb necesitar (to need). The -o ending specifically means "I need." You could say yo necesito, but native speakers usually drop the yo because the verb already makes it clear.

Why is there an accent mark on the second e in teléfono?

The accent mark tells you exactly where to put the stress when pronouncing the word out loud. Without it, the normal spelling rule for words ending in a vowel would make you stress the second-to-last syllable (te-le-FO-no). The accent mark breaks that rule, so you correctly emphasize the second syllable: te-LE-fo-no.

If I need a cell phone specifically, do I still use teléfono?

You can, as teléfono is the general word for any phone. However, in Latin America, people very commonly use the word celular when they specifically mean a mobile phone.