ABTO Software's VoIP SIP SDK allows quick VoIP softphone app development. Also you can use it when you need to add VoIP features to your software.

Quick facts about ABTO Software VoIP SIP SDK for Android:

Versions:

VoIP Video SIP SDK for Android

Standard VoIP SIP SDK for Android

Trial:

Download sample sip softphone on Google Play

Request trial SDK

Features:

All wide-spread Android versions (2.3.5+, 4.x, 5.x)

Support of SIP and RTP

UDP/TCP support
secure connection via SRTP and ZRTP;

STUN and other IETF standards (compatible with SER, OpenSER and Asterisk)

ICE support

SIP Registrar (SIP server) support

Audio and video calls

Single line support

DTMF

Status messages

User Agent name support

SIP port change

Available SIP port check

Bluetooth support

Second registration

Different networks support (WiFi, GPRS, 3G, EDGE)

Registration proxy support

Log support

Available codecs:

G.711-ALaw

G.711-muLaw

G.729

Speex

Theora

GSM

H.263

H.263+

H.264

Also you can use the following user features of VoIP Video SIP SDK for Android:

Set a custom ringtone

Use softphone with no registration (useful for calls in local network)

Communicate via IM chat

Check Call history

Store and use Contacts list

Play File to conversation

Put a call on Hold

Record a Call

Send DTMF (useful for calls to call center)

Enable/disable Video, Microphone and Speaker during the call

Sample app is available at https://play.google.com/store/apps/details?id=org.abtollc.videosoftphone