The full TEAM VOIP client has not been developed or tested on the SB1. As of now, only TEAM Express is supported and included with SB1. Remember the SB1 is designed to be a low-cost thin client device and therefore has a low memory footprint (128M), so it may not be appropriate for full VOIP solutions. To answer your question, the customer could potentially develop a VOIP solution on their own but would need to provide the UI through the shell if they need it to work with other apps while supporting the overall architecture of the device.
I understanded it will be difficult that develop full VOIP solution on SB1 and clarify it to the partner.