This is a solution for jailbroken (and unlocked) iPhone 4S, iOS 5.0.1 WITH iAP-Cracker INSTALLED! It could/should work with other devices (iPhone 3GS, iPad, etc.), but I have not tested that.
1. Delete the current version of Talkatone you have from your iPhone and from your iTunes Apps.
2. Sync iPhone with iTunes
3. Download Talkatone Version 1.2.3 from Download Talkatonev1.2.3iLuiChannel.ipa (14.52 M free and instantly from FileTrip - FileTrip
4. Mac Users: install downloaded ipa by either double click (will automatically be installed in iTunes) or drag ipa into iTunes Apps window
5. Sync itunes with iPhone to install Talkatone ipa on your iPhone
6. Use the trial version of Talkatone Premium for 14 days, then purchase for as many years you expect to live (remember: iAP Cracker has to be already installed and ON)
7. After that you can use Talkatone Premium without ads, better sound quality and use it with more than one Google Voice account.
8. According to other hackers you can even upgrade Talkatone to the latest version without loosing Premium, but I was too chicken to do it yet
Would be great to receive feedback from other users via post or private message.