Default HTC One M8 Dual SIM SIP client (VOIP) missing

Hello! The dual SIM version of the M8 does not have the possiblity to set up a SIP account with the native client of Android 4.4 (in contrast to the single SIM version). Thus I have installed the "SIP Switch Widget" from Google play store. This allows me to set up the SIP account and choose the method of dialing (asking before each call if I would like the mobile account or VOIP). Nevertheless if I choose IP-call the call is made through the mobile network. I guess that the phone app has been modified by HTC. The behaviour of incoming VOIP calls is as follows: The phone rings but I am unable to answer the call. Has anyone a fix for it or would develop a solution that the native Android client works? I would certainly pay for it because the native client drains less battery than 3rd party apps. Thanks a lot!