AGEphone Business 4+

"AGEphone Business" is a SIP-based VoIP client for iOS devices. It is designed for the business scene and not bound to any specific VoIP service, but designed to work with any provider you wish to use.
Create multiple profiles with various SIP providers and switch between them to take advantage of whatever services they might have to offer.

Highlighted Features:
-Call Transfer
-Manage multiple SIP account
-Speed Dial
-Multi-lines
-Calling Rules
-Predictive Dial
-Park hold
-Address Book Integration
-UDP/TCP/TLS protocols supported
-Voice Activity Detection(VAD)
-Speaker-phone
-STUN Support
-Jitter Buffer for WiFi and Mobile
-Multiple Codecs supported(G.711u / G.711a / GSM / Speex / SILK / Opus)
-Echo Cancellation

IP-PBX and Asterisk servers are officially supported.

■On iOS 7.1.0 devices, the notification sound continues to play even though an incoming call is accepted from the notification alert while AGEphone Business is operating in the background. Please disable AGEphone Business's sound in the notification center in iOS settings to prevent this.

■This issue has been fixed and should no longer occur on devices running iOS 11 and/or later.