Had the same problem with sound not working but the support page has the answer:

Tips and tricks
Using sipphone.com service
1. Sipphone SIP service does not support chat.
2. When configuring an account:
- enter proxy01.sipphone.com as a domain
- enter stun01.sipphone.com as a stun server(advanced settings)
- select UDP transport (advanced settings)

That got it working for me.