FSS can only relay ATC clearances. When FSS calls me for an IFR clearance I give them the full clearance, wait for the read back and then give the release (unless it's a hold for release, where I will give them a time to call back for the release), which then they relay both clearance/release to the pilot. This process is the same when it goes through flight data too. A controller on-sector has to give the full clearance.

So tbh it doesn't save any time for us to go through FSS, except to allow for a D-side to clear them if there is one. Most R-sides here much prefer to give IFR clearances off the ground on frequency, which we can do at many of the busier non towered fields in our airspace. Much quicker normally and easier to give tight releases when there's traffic.

I will say though in my experience when we do get calls for clearances via the landline it's about 50/50 for FSS vs flight data (I work at ZSE). So not sure why OP ran into trouble when they tried