Poor security to my account

I had a contract with Sprint for 4 lines and had no problem until Sprint started offering free phones to add a line.  Against my knowledge, my daughter and grandson ordered two phones (catch to drop the lines after 15 days).  They did not drop the lines and I was billed for additional lines and fees.  I called Sprint and told them I was unaware of this change and told them not to make any changes to my account unless I came into their store and showed ID.  They agreed.  After this, they allowed my grandson's girlfriend to call them, pretending to be me, and ordered two phones.  I was now paying for 7 lines instead of 4.  I called Sprint again.  I was told that if it was done online, it was my fault for their having my information.   They said  I owed them $1900.  I recently got a collection letter stating that amount.  Two weeks later I got one stating I owed $3800.  I have not used their service since December 2013.  The lady at Sprint told me they had received several requests by me to not let any changes be made to my account unless they knew my pin and answered the security question which I knew they had no knowledge of.  Sprint is a sorry company and I believe they ignore these security issues to total up expenses to collect from their customers. ,