I have never used an isp for my email, wouldn't dream of it as indeed it ties you to that one isp, and looks naff too, indeed downright gauche if you are a small or large business. I use an excellent small mail service provider.
Concerning the splitting of phone and internet access. At the moment I don't have a landline at all, don't need one and it's all more potential for quality degradation having to have DSL filters, so I don't have filters at all and no phones and no extensions. I do have to pay 'line rental' (ultimately goes to BT) for copper lines, and I switched to pay it to my ISP as then there is only one place where the buck stops and no arguing about what 'kind' of fault something is, as aside from crosstalk or some types of interference bad copper is just bad badness and it is all complete utter nonsense anyway - faulty copper lines affect DSL unless you pretend they make your 'phone' ill and choose to deal with it that way.
Re: Ronski's post, I don't do that kind of email redirect as then there are two things to go wrong so double the risk of failure. I get superb email facilities from my mail service provider for cheap money and they handle everything. Also I can send email wherever I am because I can do so through the mail-SP's SMTP servers regardless of how I'm attached to the internet, seeing as most outgoing email servers that are operated by ISPs tend to only allow users to send mail out through them if coming from IP addresses that are inside the ISP's own network, so they are totally useless if you are travelling. All a lot more hassle.
Basically I can't think of a single reason to use an ISP for email, just hassle, limitations, reduced functionality and reliability and breakage.