MizuDroid is a free, unlocked, professional SIP softphone from Mizutech.

The app doesn't include any VoIP service. You are free to configure it to be used with any SIP server or service provider.

Note1: You will need to have a SIP account to be able to use this softphone and calls to mobile/landline phones might cost you money. See your VoIP service provider for the exact terms and pricing.

Note2: The softphone might try to run in the background and request to ignore battery optimizations only if push notifications are impossible in your network, for example if your SIP server is on private IP and doesn't support push notifications.

-The VoIP phone can be used with any VoIP service provider, any softswitch or PBX, including Asterisk, voipswitch, 3CX, Cisco, FreePBX, Elastix, OpenSIPS and others.
-Compatible with all IP phones and SIP dialers such as Acrobits, Bria, Linphone, Zoiper or CSipSimple
-Works on any network above 12 kbits (3G, 4G, LTE, 5G, WiFi, others).
-Not another csipsimple/sipdroid clone (SIP client stack made by Mizutech from scratch based on Mizutech AJVoIP Java SIP and media stack)
-Free for non-commercial usage. No advertisements.

FEATURES:
-Auto adapt to your environment (device/CPU/network/server and peers capabilities)
-Minimal CPU and battery usage. File size is below 8 MB and contains all SIP signaling and media features.
-Multiple SIP accounts (from advanced settings -> general settings)
-Multiple lines (simultaneous calls)
-Push notification support direct to SIP server or via FCM
-Call divert: mute/hold/forward/transfer/conference
-HD Audio, wideband and all common audio codec: Opus, Speex, G.729, GSM, iLBC, G.711 (PCMU/PCMA),
-Enchanted audio quality: AGC, AEC, PLC, noise reduction, silence suppression
-NAT/firewall traverse capabilities (with STUN, ICE, tunnel and related technologies)
-IM (chat), video, DTMF (RFC2833, SIP INFO and in-band), Voicemail, Voice recording, Balance display, Caller ID, Number rewrite rules
-Transport protocols: UDP/TCP/TLS. Support for DNS SRV
-VoIP tunneling and encryption (optional/automatic) with auto transport selection: UDP/TCP/TLS/HTTP/VPN (VoIP over HTTP)
-Call recording and file transfer
-Presence, BLF, contacts sync
-Peer to peer encrypted VoIP media
-VoIP to native dialer integration (optional) and integration with the native contact list
-Handle incoming calls while in sleep or doze (with push notifications or running as service if push is unavailable)
...and many more

Branded softphone builds with customizations are available for VoIP service providers and companies.
Contact info@mizu-voip.com.
Details: https://www.mizu-voip.com/Support/Wiki/tabid/99/topic/Customized%20Android%20softphone/Default.aspx

Use the forum to submit any bug reports or suggestions:
https://www.mizu-voip.com/Support/Forum/tabid/92/forumid/8/scope/threads/Default.aspx

Softphone user guide:
https://www.mizu-voip.com/Portals/0/Files/Android_Softphone_Guide.pdf

Visit the homepage for more details about this SIP client:
https://www.mizu-voip.com/Software/Softphones/AndroidSoftphone.aspx