I am of two minds on this topic, on the one hand I really do like seeing business clothes being worn, it makes things feel a bit more professional. I don't have many facts but when I used to be able to wear jeans and t-shirts to work everyday I just didn't put in as much work.
Alot of this depends on the call centre you work in. I've worked in two major companies.
Company A) Very young crowd and employeed primarily people who were...how do you say...not educated to a very high level. Alot of these people were students and the churn rate was quite high.
This company flip flopped several times between dress codes but eventually ended on something near casual. People just felt more relaxed and didn't feel like it was some big chore to get dressed in the morning and didn't spend a portion of their paycheque on this. It wasn't a "career path" for most people.
Everytime they tried to implement a dress code people would get mad, satisfaction would drop, and productivity would decrease (often due to people being sent home when not meeting dress code). One of the main reasons I think was because there was no reason to have a dresscode...no clients every visited us.
Company B) Current employeer, large, very professional company, with the call centre being only a small component of it. We have casual day once or twice a month where you can wear jeans but beyond that it's pretty much dress pants and shirt everyday, (tie is optional though, as it suit jacket), this is what we consider "business casual". (Business would be with tie and jacket)
I'd say both these companies wouldn't really tough on things like veils, crucifixes, etc. The biggest problem I see (more at Company A) was people wearing sexy clothing often had to be talked to or sent home.
Another major problem with dress codes is that not everyone applies it the same, I mean..as a manager I may send home someone who works for me because of a dress code violation, but what happens when another manager let's their employee stay because of it?
I'm not sure exactly where I stand on this besides saying that the dresscode should be laid out and explained very clearly, and applied evenly and fairly to everyone, no matter gender, race, religion or sex.