ZyXEL Reach

ZyXEL Reach is the best mobile SIP client capable of acting as your desktop phone while traveling. Integrated with ZyXEL ISG series, it supports all the features business users need with intuitive behaviors. Supporting push notifications, ZyXEL Reach is the most battery efficient way to receive VoIP calls on iOS.
(Note: ZyXEL Reach only works with ZyXEL ISG series product. It is not designed for other VoIP services.)

The most popular features,
*Multi line
*Call conferencing
*Transfer and attended transfer
*Call waiting
*Voicemail Notification and programmable Voicemail dialer
*iOS4 backgrounding support
*Call recording
*Bluetooth headset support (iPhones, iPad 2 and iPod Touch 4th Generation only)
*NAT Bridge subscription for solving audio issues over 3G
*Customizable ringtones
*Quickdial, 12 favorite contacts
*Number rewriting
*In-call quality indicator (network quality, packet loss and jitter)
*Background noise suppression
*TLS support for encrypted SIP
*SIP Proxy Support, VPN Support (configure VPN in iOS settings)
*Excellent sound quality, supports G.722, G.711, and G.729.
*Native Contacts Integration
*SIP Log for troubleshooting