All iPhones sold in Mexico are sold as carrier locked and none of the Mexican carriers offer unlocking. Only Movistar can authorize the unlocking of your iPhone, but they do not offer unlocking. So, if you want to use an iPhone with Telcel, you need to sell your phone & purchase one locked to Telcel or travel somewhere where officially unlocked iPhones are sold(Canada, as an example) & purchase one there. However, your warranty & support would only be valid in Canada, if you did that.
You're wrong about the warranty being only valid in Canada, Apple products can be serviced anywhere in the world, it doesn't where they are bought.
In response to the OP the only thing I can advice for you to do is to Google search "iphone unlock", see what you get, there are ways for you to do it without having to sell it and buy it unlocked at a steep price. However, if you are willing to do that then (I believe you are in Mexico because of the carriers you talk about) you should go the Apple online store in Mexico and buy it unlocked from there.
"You're wrong about the warranty being only valid in Canada, Apple products can be serviced anywhere in the world, it doesn't where they are bought."
No, you are wrong. Unlike other Apple products that are not locked, iphones do indeed have a warranty that is good in the country of original purchases ONLY. There are very, very few exceptions to this.
ONLY the company to which the iphone is locked can unlock it ( legally/legitimately) and not all providers provide this service.