15Mbps is the hotspot speed not the phone speed. There may not be a hard limit when defining unlimited data but in the terms and conditions they do have a clause if they determine if you have been abusing it they can limit or terminate. On Visible's Reddit page some have posted that 450GB seems to be the point at which it is considered abuse and those were throttled to something like 1.5Mbps. One of the users, from YouTube, that did this was testing Visible's to see if there was an actual limit and did, had Visible remove the limit and then proceeded to hit that limit again. There have also been one or two posts there mentioning 250GB of use on hotspot falls under the abuse section, I part in terms believe that falls under the section in the terms that mentions using Visible service as a means of whole home internet replacement. There was also a 4-5 year old post mentioning getting a notice from Verizon(Visible is owned by them and must monitor excessive data use) that they will have their service stopped if they didn't stop using an excessive amount of data, the person reported using over 1TB of data.
So, even though it is implied that it is unlimited there is a threshold at which it is considered abuse and apparently you hit that threshold.
