Ok, here is my thoughts on this topic, whilst I agree with @alefattorini that this SIP tool / application does not directly relate to NS (NS is a Operating System / server infrastructure toolset and that any end-user based client application is not directly related to the functionality of the server product).
However, I have been installing and repairing end-user workstation, laptops and other devices for over 20 years. When installing new OSs on user workstations, I have always need to either install extra software or (in the case of MS Windows users) prefer to provide a medium (ie. CD-ROM, DVD or USB flash media) that contains various applications (PDF reader, archiving / compression tools, alternative web browsers, alternative office solutions and communication tools, inc. Instant messaging and IAX / SIP based client software -- I usually include some documentation that acts like a end-user 'welcome' manual that explains what is included with the medium and how to install the included applications). -- These days I try to only include open-source based applications (ie. Firefox, Libreoffice, Pidgin, Okular etc.)
As I have already mentioned above, whilst NS is a server based product and does not directly involve itself with the choice of client applications, I do believe that by highlighting some open-source, end-user products that can directly communicate with NS / existing infrastructure is a positive step for both NS and the app developer.
With that being said, I want to examine this SIP client app a bit further. I have had a lot of experience with both free (freeware, open source, shareware etc.) and commercial SIP clients (some good software and others not-so-good apps).
@ghost, I am wondering if you are going to incorporate both SIP and IAX protocols into your project?
One thing that I have always had a problem with most SIP clients is that there is no integration with existing 'phone books' / contact lists. Considering that NS (and other non-NS) administrators incorporate groupware applications (ie. Nextcloud or SoGo) and the fact that these applications can be used to store CardDAV / WebDAV information, will your SIP client be able to access SSL CardDAV (or maybe, extra LDAP) information?
Another thing that I have found with a lot of desktop client software is whilst it is possible to add multiple SIP accounts, the software will only allow for a single account to be active at any one time. I do hope that you will allow for multiple account access.
I am interested in knowing what codecs your are going to be providing and if you are going to include G270 (I am curios, how / where to purchase the license agreement for this codec)?
I am also interested to know what platforms / OSs you are intending this project to be used with?
As said above, I have used a lot of SIP based client software in the past, some of my favourite tools inc. CSipSimple (Android), SFLPhone (Linux, OSX, Windows -- Project is now defunct but I am using it on my daily workstation incorporated into SUSE / KDE GUI) and JitSi.
Personally, I would like to see a client software that incorporated both SIP, IAX and XMPP / Jabber. With options for CardDAV, STUN, SMS / MMS (?) and WebRTC access.
Final note about funding, I agree with @alefattorini and after reading your help.nextcloud.com post, I am wondering if this should not be a 'kickstarter' styled project?