For use ins:
A good SIP phone library accepts stimulus input, one version will accept the keys found on a POTS phone, and construct "star-code" messages. This is a great way to add an RJ-11 connector to your device to turn it into a SIP-analog telephony gateway. Your customers would just hook up their existing analog phones and could then make VoIP calls.
If you choose to use a licensed in a standalone SIP phone, you can send it stimulus commands and not bother with star-codes. In this case, you can implement a two line phone.
A good softphone is available ported to the most popular mobile phone operating systems.
An embedded SIP softphone is ideal for cost and memory sensitive applications:
A quality softphone offers a comprehensive set of voice phone functions including: