There are 3 main types of suppliers:
Suppliers who will approve you without any issues!
Suppliers who will approve you without any issues will often set you up through email or a "become a dealer" page on their website.
Suppliers who will take a little longer to approve you.
Suppliers who will take a longer time to approve you will require a phone call and its mainly because the owner or person in charge of new accounts wants to verify that you are who you say you are and are confident enough with a complete marketing plan you can tell them about in real-time so they know you aren't either a fake or a newbie.
Suppliers who won't approve you for a very long time if ever...
Suppliers who won't approve you probably either sell direct, have maxed out their amount of dealers they are comfortable with, or have had bad experiences with dealers providing end consumers with bad customer service. These are usually the biggest brands in the industry and although they will take more time to get approved with, they are worth applying for and marketing because their products probably sell really well and also have a ton of people looking to buy them.
Make sure to keep notes throughout the supplier approval process within the suppliers tab of your drop shipping master spreadsheet. My average approval rate is with about 60-80% of the suppliers I found on my competitors sites. Out of those about 50% approve us within 3-6 months. After that, we follow up every 3-6 months to check back and keep building our relationship. Eventually we get approved with almost all of them, especially if our competitors already have.