"Pago mi la cuenta de teléfono a mensual."
I pay my telephone bill monthly.
10 Answers
I hardly ever hear anyone use mensual unless it's an out of the ordinary type thing. Maybe someone got a ticket and they have to pay the fine mensualmente, but even that's an oddity.
Obviously, most people, most of the time, are talking with friends/acquaintances/co-workers and they share a lot of common experiences/common knowledge. In such cases it would be, as you said, unnecessary to specify "monthly". However, some people (especially those learning another language) find themselves in conversations with people who have very little knowledge (or, lots of knowledge but all gained from watching Hollywood movies) of "how things work" in the USA. I have found myself, on occasion, explaining things that most Americans would consider to be "common sense" to a foreigner and seeing him be amazed at such "weird customs".
As a very crude example, there are parts of the world where you couldn't tell a story about getting a traffic ticket for "running a stoplight" because they have no traffic lights. Similarly, if you were talking about telephone bills to someone in some small village in (wherever) who had never used/owned a telephone you might need to supply more explanation/background than when talking to your spouse/brother/etc.
Here is a thought. When you are a beginner it is best to learn to structure sentences properly and completely. Later on you can shorten your sentences and learn all the slang, etc.
Yo pago mi cuenta de teléfono mensualmente.
Any conversation may demand that you would be very specific in your speech. Details are often necessary to express how we feel about a particular statement. e,g, " Yo nunca dejo de pagar mis cuentas, Yo pago mi cuenta de teléfono mensualmente."
I think it really does depend a lot on the country where Spanish is being spoken, I just got back from a trip to the DR and they are always saying "mensual" and "mensualmente" - but, I rarely here is spoken by the people here in the states - most of whom are from Mexico.
There are alot of people that are quite fond of saying I am a slang speaker, I'm from the street, etc. I'm not...pues...si soy de la calle, pero como yo lo hablo es mas sabroso, mas normal, tiene el ritmo mas natural que ellos ya saben. 
My english is perfectly fine, why would anyone think my spanish is any different??? I'm just saying. Lo que digo a mis amigos en este pagina es como funciona espanol en mi propio mundio, como se va en Mi Tierra. I'm the Voice Of How Them Spanish People Speak When They Are Alone Or At Lunch. Anyway, No one really uses "mensual". You are correct to have fear of it because no one In My World, Como Mis Conocidos, lo usan. It's an Out Of The Ordinary type of word that you use for an Out Of The Ordinary type of thing. The closest you are going to get to in (In My World) is mensualmente. Example: I pay my rent mensualmente. I pay my car note mensualmente. My wife gives me the Good Loving mesualmente. That's how that word works en Mi Propio Pueblicita.
I'm forever saying to people in my posts that I'm only talking about Where I Live. I say that forever. How I use spanish is a result of Where I Live....the combinations, the flavor, the ritmo...I'm forever saying that. I don't know what goes on in other parts of the world, I live Here. I was just trying to tell her how we all speak spanish here. And here...generally the style here is we almost never use "I", never speak in full sentences, (I've never heard anyone who really does, except on the news) and around here we pay our bills by the month. Tonces....he pegado la cuenta. Really, no one would say "mensual" unless it was an unusual occurance of something. @Izanoni...They don't do that around here. No one I know has a phone (or anything else for that matter) that they pay by the week or every two weeks.
Tengo miedo de utilizar la palabra 'mensual' en una declaración. How's that for use of the word of the day?
I read my comments to you and I thought it came across in a way that I didn't want it too. I don't like to spar, either. I was just saying. It's so tough talking on the computer...what with the lack of visual clues, etc. I wasn't sure what the question was exacly...I hardly ever hear anyone use mensual unless it's an out of the ordinary type thing. Maybe someone got a ticket and they have to pay the fine mensualmente, but even that's an oddity. One of those words that you know when you hear it, but don't actually use.
Hi John
@Izanoni...They don't do that around here. No one I know has a phone (or anything else for that matter) that they pay by the week or every two weeks.
I wasn't trying to spar with you. My first comment was directed to you, but I was just trying to say that I think the only reason that Jack started this thread was because he was trying to practice using the word of the day, "mensual." I would agree with you that in practice most people would not go through the trouble to say, "my monthly bill" in Spanish (or in English for that matter), but there is nothing wrong (as far as I can tell) with saying or writing it that way, especially if it is just for practice, and if he is just trying to get a "feel" for the word.
As far as my second comment was concerned...that was not directed at you (sorry for the confusion). It was directed towards Guillermo's comment regarding "some countries," and I was only trying to add that the US also happens to be included as one of "those countries" where this type of phone service is available.
I definitely agree with you that monthly payments are usually how most people here pay their bills, and I also agree with you that in most cases people wouldn't call them "monthly bills" just for the sake of brevity (a subject I apparently know nothing about judging by the length of my response 
).
At the same time, if someone were to speak to you about their monthly bills or their cuentas mensuales I am sure that you would not look at them as if they had a leg growing out the side of their neck just because they used this adjective.
He pegado la cuenta. Everyone knows bills are monthly, so, in spanish, you generally don't state the obvious.
