Questions about subject-to purchases

[Indy Smith]

Hi everyone, 

Can someone please clarify a few things about subject-to for me? 

I understand that you essentially take over the existing mortgage from the seller, thus "subject-to the existing mortgage". 

Does the buyer have to qualify for the loan with the sellers lender? Or does title just transfer and then the buyer is responsible for the payments?

Does the lender ever call the loan due when title transfers? 

I ask because I have an opportunity to buy a property this way. The loan balance is $75k, the house is probably worth about $120k. I recently moved and have started investing in real estate full-time so I would not be able to qualify for a conventional refinance if the lender called the loan due. 

Thanks in advance for any responses!