It is true that Google voicemail picks up after 25 seconds (and you can't do anything about it). However, is your concern that it goes to voicemail too quickly (faster than 25 seconds); or that there are too few rings in those 25 secs?
If it's the latter (too few rings), you may want to play with Ring settings in applicable Ring profile. Those settings (like, 60;(.3+.2,1+.2,.3+4)) look Greek to me, and the documentation looks Swahili... but they can be tweaked!
