Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to call your husband's phone when he is on a phone call already, as it goes straight to his voicemail. If your husband has an iPhone, he can check the call waiting settings at Settings > Phone. Otherwise you will need to contact your carrier about the call waiting feature on their network.
Call forwarding, call waiting, and other call features on iPhone - Apple Support
It is possible that your contact may have been blocked either with the carrier who you can check with, or on his phone accidentally. Have your husband check on his phone to see if you are blocked with this article.
Block phone numbers and contacts on your iPhone, iPad, or iPod touch - Apple Support