Thank you all for contribution,
as "wroot" adviesed I send a PM to "Dele" and still waiting for his respond.
Don't you think let dedicated SIP server (eg. kamailio) serve all VOIP capability is ok? Has you ever tried it out?
I read some articles with the topic around XMPP vs SIP or XMPP Plus SIP and they almost have encouraged to do separate things (Messaging-Presence and VOIP) with separate protocols and servers, what do you think of this opinion?
I haven't work on Spark client, I have developed my own Android Jabber Client base on aSmack library.