Emtelle Group, which has plants in Hawick and Jedburgh, makes plastic piping which encases underground fibre cables.
After working with BT for more than 30 years, it has secured a four-year contract extension.
Emtelle said the move would allow it to invest Â£10m in its sites in the Borders, which support about 300 jobs.
BT said the company had provided it with more than 50 million metres of duct, sub-duct and blown fibre in 2014.
It is a key supplier in BT's roll-out of high-speed fibre broadband across the UK.
Mads A Hogfeldt, chief executive of Emtelle Group, said: "Today is an exciting day for Emtelle. This reinforcement of our long-term relationship with BT will help us continue to grow our business and provide quality employment for more than 800 people, including 300 at our two manufacturing plants in Scotland.
"The relationship we have developed with BT over 30 years has provided a very stable platform for us to make long-term investments, drive down costs and develop business in key areas such as export to overseas markets.
"We are very proud to be helping Openreach to deliver one of the most significant civil engineering projects in Europe, creating a huge, high-speed digital platform for Scotland and the UK which will benefit generations to come."
A cable manufacturer based in the Borders has won a multi-million pound contract with BT Openreach.