I changed to optus internet and phone and always having problems?

4 months ago I changed to optus and signed up for a 2 year contract for $109 including 500 gb of internet,line rental and free calls to all landline and mobile phones..The trouble is the internet is always dropping out and more often then not runs slow with speeds less than 1mb. when rarely working properly I get speeds of 7 mb.Have spent hours on the phone with indians and they blame my phone line,saying it is telstras fault although the line has been serviced 3 times and I have spent $100 on servicing internal phone connections.Even if I tried I could not use 500 gb as the service is too slow,which is a problem as I rent rooms out to students.I live on the sunshinecoast in qld.Are they telling the truth and is it inferior phone lines or is it there servers cant handle the influx and they are advertising a product they cant supply.