Well, I’ve finally found out why I can’t make calls when my HTC Tytn is in HSDPA mode.
It turns out to be an issue with the Radio Access Network (Base Stations, I assume); on all Ericsson equipment, HSDPA mode takes exclusive use of the radio connection, and it’s not possible to fit in a voice connection on the same radio. As a result, if your phone is in HSDPA mode, then any incoming calls will be diverted to voicemail, and any outgoing calls will be greeted with a ‘beep beep beep’ failure message.
Since Vodafone UK use Ericsson equipment for their RAN (Radio Access Network), it means it affects them. Also I assume any other operator using Ericsson 3G network equipment. T-Mobile UK use Nokia, and they have no such problems. I also discovered that while roaming abroad on certain networks, I could make calls while in HSDPA mode – presumably because they weren’t using an Ericsson RAN.
As for a fix? Apparently, no time soon; there are posts of Vodafone being aware of this from April.