If I were to hazard a guess, I would say your ‘line’ is ( in part ) what is commonly called an ‘Ali’ , or aluminium cable , although that is an over simplistic term , they are somewhat inferior to a 100% copper pair cable , but they are far from incompatible with ‘broadband’, they ‘may’ deliver a slightly reduced performance when broadband is delivered over them ( compared to a copper cable ) but it’s nonsense to imply aluminium = no broadband, in some circumstances there is no reduction in performance at all.
In the 1970’s the price of copper was such that some phone ‘cables’ were made from this Ali/copper hybrid material , and have proved to be less reliable than purely copper cables , and in comparison to a copper cable can in some circumstances, provide a reduced performance when it comes to broadband ( something that couldn’t be conceived back then ) but the phone network cables , both copper and Ali were never designed to use the sort of frequency’s that broadband uses, but it’s a relatively minor reduction in performance….however Ali cables certainly certainly do have a bad reputation with external engineers , and perhaps whatever ‘engineer’ you have spoken to , with the inability to fully repair a cable fault , has oversimplified this , and left you with the impression that broadband simply won’t work over this type of cable , but that is wrong, there will be hundreds of thousands of customers ( if not millions) using cables that are in part ‘Ali’ and have a perfectly usable broadband service over it .
It was also commonplace in the 1970’s to bury cables ( both copper and copper/aluminium ) directly in the ground , so if one of these cables becomes unreliable, ( not incompatible) it’s not a simple job to replace it , which would normally be the preferred repair for a cable that has a higher fault rate than normal.
If you had broadband, but it performed poorly , the chances are there was a defect in that cable , rather than any inherent incompatibility, and if that cable were in a duct no doubt it would be replaced with a copper cable but if DIG ( direct in ground ) that is a much more expensive proposition, and wouldn’t be considered, if all that would be gained would be a smallish percentage improvement in broadband speed ( say something like 10-15%)
Getting back to the original point , even the worst performing Ali cable pair can host a broadband connection that only needs to deliver 0.5Mb speeds , more than enough for Digital Voice, which was your original complaint
