Hi,
I am using “VoIP.ms SMS” app for this purpose: https://play.google.com/store/apps/details?id=net.kourlas.voipms_sms
It is OpenSource: GitHub - michaelkourlas/voipms-sms-client: Popular Android messaging app for VoIP.ms, a Canadian VoIP provider
Maybe Voip.ms could contribute somehow to it. A dedicated app is truely the best way to go forward. 
Kind regards,
Alexandre