If this is to be truly mobile, hardwired SIP handsets (over VPN or not) are not an option. In fact having to carry anything in addition to a mobile phone is not an option.
As far as possible having the handset connected into the PBX (via SIP most likely) gives the best functionality options. There are times that this will simply not be a option, thus DISA or similar functionality is considered too.
4 linphone installations constantly connected (when on Wireless) and in use.
I want the option (if it's appropriate) to both contact and be contacted anytime anywhere. I want good quality voice.
I'm fiercly suspicious of anything given away for "free" so Skype and similar services are not really options here. I want to be in control of my calls and recordings.
Using a mobile phone handset (Samsung Galaxy S3) and DISA / SIP to come as close as possible to providing ubiquitous telephony.