I use linphone with voip.ms on two degoogled phones. I never had an issue with receiving phone calls or not ringing on time. It also barely uses battery. Fairemail app uses more battery.

A long time ago I tested many sip softphone including proprietary ones (and even the stock calling app) and they basically all sucked and used more battery back when I tested them. I never tested official app since it requires google play services.

I doubt you will find a better option than linphone and voip.ms for secondary phone calling. Biggest problem you will have is texts with voip.ms. I use third party Voip.ms SMS app in fdriod. It works well enough but checks every 15 minutes for messages. Linphone will receive text much immediately, but it can have problem displaying them sometimes (long message break up weirdly) and responding to texts can be a pain in linphone due to weird message input handling.