Can cat6 be used as a 1:1 replacement for telephone cable ? Ive seen it done multiple times and its worked fine, but can it be considered a 1:1 replacement or are there some downsides of doing it ?
A higher category cable can be used for a lower category cable. Ordinary telephone cable is Category-3 cable.
The downside is that simply replacing one category cable for another can be very misleading. Simply having cable at a category is meaningless. All the cable components (connectors, patch panels, etc.) must all be the same category, and they must pass the category test suite after installation to be considered in that category. Installation can be challenging for higher category cables, and I have seen experienced installers frustrated with Category-6 cable installation (getting it to pass the Category-6 test suite).
You can also have problems because the cable and wire diameters can be larger in the higher cable categories. This can present problems with the cable path, and cause poor connections if lower category connectors are used.
When a cable vendor installs cabling, you need to get each cable tested for its category suite, and a complete report should be supplied to you after installation.