I built offices and I want to have 6 numbers on one phone . It is possible to do that?

Yes - if you mean 6 phone LINES.

Several ways, you can rollover, use a KSU(Key Service Unit/hone System).
First ones I did 20 years ago with 6 lines on a cordless phone, was with A Vodavi 616(6 incomming lines/16 stations), and a SLA(Single line Adapter) that allows a regular phone to be used on a phone system.
Instead of getting a dial tone, you would get a tone and 71 would get line 1, 72 would get line 2, and so on, you could transfer calls, Page, anything a system phone would do, only cordless, and I would use a Engenius Phone that would allow long range multiple line capabilities.
Many systems allow the use of regular CO type phones.