My day job involves 5 hours on the phone at a time. How should I go about protecting my voice? I have to talk to old people, so I can't always speak quietly.
There are two resources from the British Voice Association you may find useful:
(1) Voice disorders and the workplace (2) Take care of your voice
Resource (2) includes advice on eating and drinking, keeping hydrated, etc, and from my experience drinking plenty of water and if possible giving yourself many small breaks to rest your voice is probably most relevant.
Resource (1) includes suggestions for making the workplace more voice-friendly, for example personal amplification. In your case, I wonder whether the telephones have any settings for amplifying your voice so you don't have to shout? The LARY project mentions telephone amplifiers for outgoing sound http://www.lary.org.uk/voice-equipment-/equipment-overview so you may want to ask your employer what they have available.
Resource (1) also raises the issue of the amount of background noise (could you work during quieter periods, or ask to be somewhere where you're not also speaking above background noise?), and varying your workload if possible (any chance of alternating your phone work with back-office work?).
This article from Marketing Week includes specific ideas/suggestions for protecting call-centre operators (for example, shorter sound-bite-style scripts), and you may want to raise some of these ideas with your employers if you're concerned: http://www.marketingweek.co.uk/home/voice-concerns/2021792.article
Finally, you may specifically want to seek out voice/singing training to ensure that you are using your voice as efficiently (and with as little undue effort) as possible.