How are cosigners impacted by my other lines of credit?
I have a cosigner for a school loan that is concerned about my other financial ties affecting them. For example, if I have a phone bill with someone with poor credit, could that somehow affect the cosigner? It is my understanding that the cosigner is only connected to me financially through the loan itself, and that their credit will only be impacted based on the payment of that loan.
Your assumption is incorrect. If someone signs a loan and any of the parties on the loan do not make timely payments, then all of the parties who signed or co-signed the loan will have their credit rating affected and are each 100% liable for the total amount of the loan. That is why you should never co-sign a loan with anyone but a spouse, and sometimes not even a spouse, since if the other person doesn't pay then you are fully liable in all aspects including interest, fees, penalties, and the negative impact on your credit rating.
Sometimes you have to just say no, even to a close friend or relative.
Your understanding is correct. The cosigner is only responsible/affected by the loan they actually signed for. Your other debts and bills have no effect on them or their credit.
However, not that this will be the case, but so you have the information, if you fail to make payments on the loan they cosigned for, they will be responsible for it, and if that account goes to collections, it will negatively impact their credit.
I hope this helps!