Yes, I'm guilty as charged!
When you work in a professional office and have to talk to various different people everyday ranging from the general public through to company directors or very high up medical professionals, it "doesn't do" to answer the phone in a "common" or "relaxed" accent, so I believe most people in those scenarios DO have a "telephone voice" because you're representing the company/organisation and it projects a more professional/knowledgeable image.
Yes, if it's a friend/colleague or someone you know well on the other end of the line, your accent might change back to your "home voice".
Even though I've not been in an office environment for several years now, if I have to telephone anyone other than friends/family my (Solihull) telephone voice comes out because I sound a lot more professional than my home (Solihull crossed with Birmingham) accent might portray and I believe people treat me better too.