I recently switched over from Vodafone to Telecom and to much of my suprise, I found the voice qiuality on Telecom 027 significantly better. Suprising because I had heard and read a few times that GSM quality was superior.
I would imagine signal strength and phone would play a role in voice quality, but I am very happy with the clarity on 027.
Strange, I just switched from Telecom (Hyundau tx95c) to Voda (Nokia 6600), and have found the voice quality a lot better on Voda. I guess Nokia do have a lot of experience with this.
One thing I do notice though, is that on Telecom, you get a bit of warning before the call drops (quality gets bad, one person can't be heard), so you can try to find better reception. With Voda (or perhaps Nokia), I find that the call stays pretty good but then might suddenly get bad and then drop within 5 secs of getting into really bad reception.
I've also just switched from Voda to Telecom. I found that the CDMA has better voice quality and also the it filters a lot of background noise as compared to GSM network. Is this related to the network or the phone itself. Currently I am using a samsung N181.
The background noise filtering is network related and is fairly consistent throughout the current phones.
When I changed from my GSM Nokia to 027 I did notice a significant improvement in voice quality. While my GSM Nokia had a tendancy to cut in and out if I started getting into a bad reception area I have not noticed this at all with my 027 (I am currently using a Sanyo 8100 and a Hyundai TX95c)