"Mr. Pérez has a cell phone."
Translation:El señor Pérez tiene un celular.
"El senor" is formal and is used when addressing an older gentleman or a stranger whose age you are unsure of and are therefore trying to show respect. La senora is formal and is used when addressing an older woman or a stranger whose age and/or marital status you are unsure of and are therefore trying to show respect (in English we use "Mrs" for a married woman and "Ms." for a marital status for which we are unsure.). "Senorita" is not formal and is used to address a young, unmarried woman (in English, we use Miss without a period. ).
"Un" is used ahead of a masculine noun, while "una" is used before a feminine noun; both mean "a", as in "una mujer" for "a woman" or "un restaurante" for "a restaurant". If "el" (masculine nouns) or "la" (feminine nouns) is used in my above examples, it means "the woman" and "the restaurant".
Ask someone in the UK what that device is they're staring at all day long and they'll say 'it's my mobile'. They wouldn't say 'it's my cell (phone)', unless of course they're visiting from the USA. No-one living here calls them 'cell phones'.
When talking about someone to other people that you'd call "Mr." or "Mrs." directly, you use "El" or "La". It's a general Spanish rule, like with a noun, where you have to have something in front to make a correct word/statement. In this case, in the English translation, the "El" or "La" are left out.