Any experts on cordless phones or multiple phone lines on here? Please say "Yes!"?

I just want to make sure that I am shopping for the right product. We have a dental office with 1 phone#, but we have a rollover line. Our fax has it's own# which "was" what we called line 3 but as of today a jack was installed just for the fax, we can dial out on all three lines though on our 4 line corded phone systems. I want to buy cordless phones and a new fax. Can't I just plug the fax into it's own jack and just buy a 2-line cordless phone system with (multi-handsets) to use instead of our 4 line corded phones? We do not have 4 trunks coming into phone system. Again, one main line(one number) with a secondary rollover line and separate fax number. Two more things-I am not planning to replace the lab phone (which is equipped to handle 4lines) so can I use the 2 line cordless phones in the office while having a 4 line phone still in use? Last thing-Can anyone tell me if 6.0 ghz would be safe/private enough to use in a health care setting. I cannot have outsiders hearing protected info. Sorry to sound so redundant and clueless, but I just need to confirm I am doing the right thing here. Thanks so much!!
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is what is currently in place a regular phone, or is it a system phone? (does your office have a PBX or phone system? if you have a PBX or phone system, then the jacks are going to have to be wired so that you can use the regular 2 or 4 line cordless phones... there is no such thing as 6.0Ghz phones... there are DECT 6.0 phones, which operate at 1.9Ghz... they are plenty secure.... they transmit in digital between the handset and base, and utilise DSS, FHSS or some other form of encryption to prevent eavesdropping... as well as the fact that most radio scanners are only capable of picking up transmissions upto 1300Mhz (1.3Ghz) EDIT- you say you have a phone system - thru the phone company, so i am still not quite sure if you have a system or not... what brand/model phones are in place right now? (might be better off PM me, as i might forget to check back on this question)
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