Didn't see this forum topic at the time but i had the same issue as well on my brand new Desire s over the past few weeks.
Got a contract on O2, first time using an HTC handset or anything else running android for that matter, first week i get a lot of people telling me "your phone was off", which is strange as i live in a good area for signal and especially on my desk where the phone will usually sit (you can even see the cell tower from the window). Back to the shop where they changed the sim card and when i still had problems, the next day they gave me a new handset.
Kept an eye on it for the next few days, same problem kept showing up, if i left the phone alone for an hour or more, there was a good chance the signal would drop and wouldn't come back until i made an outgoing call/text. Every morning it would do the same also, i could even sit there and browse the net over wi-fi for 20 minutes, calling the phone would result in voicemail.
Both phones i had prompted me to update firmware, something that i would expect after opening a boxed phone but now i see the update renders the phone almost useless (as a phone). I went back on the 13th day of my contract and cancelled, i can't say i'd go for another HTC handset in the future. Looking at the Galaxy S2 now.