Procore - Procore is the world's most widely used construction project management software. Easy to use, mobile platform with unlimited user licenses.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Corecon - Corecon offers integrated estimating, project management, and job costing for small to medium-sized construction companies.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
SummitVista.io - Summit Vista end to end short and long term property management
Sipdroid - Free and open-source SIP/VoIP Client