Capacity will be expanded from 500 000 to 800 000 under the programme to be financed by the loan, Econet said on Thursday.
New lines will be released in October.
The company currently has over 470 000 customers.
The loan was secured from the Cairo-based African Export-Import Bank and would be used to import equipment allowing faster GPRS and third-generation services.
Econet, which is part-owned by South Africa-based Econet Wireless International, said the loan would be serviced through the company's foreign currency earnings.
Econet competes with two other operators, privately owned Telecel Zimbabwe and state-run Net*One.
Foreign currency shortages have hamstrung network expansion and growth in Zimbabwe's mobile phone sector, capping penetration at around 5% of the population, compared to 70% in South Africa and around 40% in Namibia.
In Zimbabwe, crippled by an eight-year recession, customers wait months for mobile phone SIM cards to come on the market, and most are forced to buy them at nearly 10 times the official price on a thriving black market.
Foreign banks have largely been unwilling to lend to Zimbabwe's private firms after lines of credit from international donors like the International Monetary Fund dried up in 1999 over policy differences with the government.