Single tel. extension for multiple phones

[MohammadAdilRaza47738]

i have a hospitality project with 1500 extensions. 

i have 2 tel sets in each apartment and the owner needs to have only one extension no per apartment

so that when you call the extension both telephone sets will ring at  the same time.

how can i achieve this  

what is the maximum no of telephone sets that can work on one extension since i also have small houses which have 9 telephone sets.

[Jaime Valencia]

You would need to start by explaining what products you're using in your environment/deployment, and not just explaining what you want to achieve while providing zero details of your deployment.

[Nithin Eluvathingal]

as @java  mentioned to have a proper solution provide us the details of call control platform.

based on the number of extensions mentioned I hope this is a CUCM deployment. you can use shared lines.