Ubiquitous open source telephony
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.
- SIP: Using a SIP client is a obvious choice, CallerID presentation and routing manipulation are possible to their fullest extent. I've tried the built in Android SIP client, but found the implementation on my devices to be unacceptable. I've settled on LinPhone for a number of reasons, most significantly the availability of the source code means I package an APK that can be completely autoprovisioned allowing a much wider test base. See also acceptable mouth to ear latency. Download the APK I'm using.
- DISA: Latency on mobile internet is incredibly unreliable, and when the latency is acceptable is isn't consistent.
4 linphone installations constantly connected (when on Wireless) and in use.