Looking at it from both sides (provider and customer) you can see how telephone support is important, especially in the case of "my service is down", as raising a support ticket may sometimes not feel like it will be given the urgency it needs (from a customers point of view).

I'd happily accept a telephone support call from a customer but if it's more than a "my service is down" then it's going to need investigating and details will need to be exchanged and I feel that is better done via a support ticket that both parties can refer back to, especially if it's not a " 5 minute fix".

I'd be a little hesitant if the provider did not offer any kind of telephone support, as a customer this would be a bit of a red flag (though I understand why some do not).

For sales, absolutely there should be a telephone option, as PureVoltage says, talking to customers and getting to know them is very valuable.