Listen! No one is really busy, it's all about priorities that in their life where do you actually exist and how much do they value you then no matter how much busy they get they will always respond to your call and texts and if you are just an ordinary person for them like others then they will always look for an excuse to tell you for not being there picking your call and responding to your texts.
Image Sources taken by me
Come-on man this is a digital era and we all have our cell phones in our hands all the time so what do you think that how much it takes them reading your texts and responding back? I guess not more than some seconds or a couple of minute but if you are important for them, they count you in their priority list then they always will respond back as soon as possible instead of ignoring and later on giving some lame excuses. So don't let yourself being fooled by them.