"Ring Time" is extremely inaccurate

I have a Ring Group (method = ringall) with 4 extensions. I set the Ring Time to 20 seconds. However, the actual ringing time is 35 or 40 seconds. If I change it to 13 seconds, it is about 20 seconds of ringing. If I change it to 30 seconds, it is over a minute of ringing.

I tried settin all of the individual extensions within the ring group to the same ring time as well, but that did not seem to change the results. The real ringing time is still much longer than the setting.

So, I think something in the calculation of Ring Time is incorrect.

Are these phones on an Linksys SPA1202 or something similar?

What phones are they? I does not necessarily have to be a FreePBX or Asterisk issue, it could be the phone or adapter if used.