This isn’t a fix for your specific issue, but since you are already at Asterisk 1.6, have you thought about upgrading to Asterisk 1.8? Since you already presumably have iksemel installed, the process isn’t that difficult - generally it would go like this, using the best build for Google Voice currently available. I do strongly recommend that you do a full system backup first, in case something goes wrong (and be sure you know how to restore from that backup before proceeding). If things stop working, the only advice I may be able to give you is “restore from your backup”, so make sure you have one.
If any part of this doesn’t make sense to you, you probably shouldn’t attempt it.
First, make sure you have some dependencies:
yum install gcc make gcc-c++ ncurses-devel libtool subversion mysql-server mysql-devel libxml2-devel libtiff-devel php-pear php-pear-DB php-gd php-mysql php-pdo perl-DateManip mod_ssl libcurl-devel curl-devel
You may already have most of those, but they will be updated if necessary, and anything you don’t have will be installed.
Then to do the upgrade, using the latest current build (yes you do want the rc1 version shown unless there is something newer, because it fixes a couple of serious bugs):
cd /usr/src
wget http://downloads.asterisk.org/pub/telephony/asterisk/asterisk-1.8.2-rc1.tar.gz
tar -xzvf ./asterisk-1.8.2-rc1.tar.gz
cd asterisk-1.8.2-rc1
./configure
make menuselect
(Stupid forum software insists on truncating that wget line - the end part should look like this: /asterisk/asterisk-1.8.2-rc1.tar.gz )
At this point you will need to go through and make sure that all the modules you need are checked. In particular, for Google Voice you will need chan_gtalk and chan_jingle (under Channel Drivers), and res_jabber (under Resource Modules). You will also need things like app_mysqp, app_saycountpl, cdr_mysql, format_mp3, and res_config_mysql (under Add-Ons), and you want to look in the Sound Packages sections and select the ones in your language(s) and for the codecs you plan to support. Don’t just check everything because then you’ll get stuff you don’t want and that might interfere with your mode of operation. Other than what I’ve mentioned above, most or all of what you need should already be checked. If you find some function doesn’t work, you can always repeat this process (from ./configure on) to add whatever you need.
Then to complete the process:
make
make install
This would probably be a good time to reboot the server, or at the very least restart Asterisk. Check to make sure everything’s working as you expect (except for the Google Voice connections). If it’s not, time to use Google to see if maybe you’re missing a module or something. If you just can’t figure it out and it’s something essential you’re missing, you can either restore from your backup, or go back to the /usr/src directory and find your most recent 1.6 install, and reinstall that.
If it works, then set up Google Voice using the Asterisk 1.8 method. The site you got your original setup instructions from may have separate Asterisk 1.8 instructions, but if not there are other sites that have them. Use the Google, Luke. 
I only mention this because it may be the quickest way to solve your problem, plus it will give you a better way to connect to Google Voice.