There are currently around 33,000 new numbers available to allocate in the 01273 dialling area. The level of demand means these numbers would be forecast to run out by early 2015.
The number of communication providers offering services in Brighton and Hove has increased significantly over the last 10 years, leading to more competition and cheaper landline bills for millions of homes and businesses.
But it has also led to increased pressure on the supply of new numbers.
The proposed change would require people in the 01273 phone area to include this code when dialling a local number from a landline.
Ofcom is proposing to implement the measure on October 1 2014 in Brighton and Hove. The measure was implemented in Bournemouth, Christchurch and Poole in November.
A spokesperson at Age UK Brighton and Hove said: "Any changes like this can lead to disorientation for the elderly. We deal with many residents who are housebound, and their telephone is their lifeline.
"We hope in the consultation period Ofcom make sure they do not digitally exclude the elderly, and make sure they contact them via post or phone."
Muriel Elms, 99, said she would not support the new changes. She is currently housebound after several recent heart attacks, and her phone helps her stay in touch with the outside world.