The SIP application is independent of The spreed and spreedme apps already implemented into nextcloud.
SIP depends on a core engine , say asterisk Kamailio Elastix…and it allows you to make tel calls from wherever you are on the move. Tel calls using your corporate landlines so your clients will not be able to recognize your actual location.
The sip app I am looking for should be able to work with asterisk version 13 and above.
I did not want to go into much technical details as I wanted the developer to propose or to negotiate a solution or a design.
There are many ways of doing it, it could be WEBRTC based or PSJIP or normal SIP protocol.
This same SIP app can also support Video and Voice as well.
Half of the work is done in github
JSSIP, ctxsip, sipml5, doubango and Janus are some examples.
In my opinion JSSIP (Voice and Video , webrtc based) as well as ctxsip (webrtc, voice only) could be the best candidates and the easiest to implement.
My budget depends on the final outcome , I am looking for a fair and decent proposition.