Internet/SIP/digital phone instead of ATA?

I have four phone numbers on two analog lines:
Number A - business
Number B - fax (Ident-A-Call number on Number A)
Number C - personal
Number D - spouse (Ident-A-Call number on Number C)
I would like to convert these to VoIP. I've asked for suggestions for the best ATA at: viewtopic.php?f=14&t=16970&p=66482#p66482 .
Now I'm wondering if I should simply get Internet or SIP/digital phones instead of an ATA since my spouse and I live in a small place and we only really need the phones in one room (my home office). However, I'm not familiar with Internet or SIP phones. Can anyone suggest if this is the better way to go and if so, which Internet or SIP phones would be the best?
I currently have a Nortel Venture analog phone that handles multiple lines. It's easy to conference two lines together and it has a nice speaker and microphone. Is there an Internet or SIP phone that is comparable to this? (This is a nice-to-have, but not a must-have.)
More info on what I'd like to set up:
When I make outgoing calls, I want to show a different number between my business and personal calls. Therefore, I'm thinking of forwarding Number B to Number A and Number D to Number C. Then I'll buy Unlock Keys for Number A and Number C.
For now, I'm not concerned about my fax machine being able to detect the incoming call, because I rarely receive faxes. When I do get a fax, I've just talked to the sender and I will manually get the fax machine to answer the call. But I do want to keep my fax phone number, in case the situation changes in the future. Am I correct to assume that if I want to use my fax machine, I will be forced to use an ATA and ditch the Internet/SIP phone idea?
When people phone for my spouse versus me, we would like to know the difference. With our analog landline, my spouse's phone number is an "Ident-A-Call" number, which means that calls to her number will ring on my personal line, but with a different ring pattern (2 short rings and 1 long ring). Can the Internet/SIP phone be set up so that the when callers call Number D, the Internet/SIP phone will ring with a different ring pattern or ringtone?
Number A - business
Number B - fax (Ident-A-Call number on Number A)
Number C - personal
Number D - spouse (Ident-A-Call number on Number C)
I would like to convert these to VoIP. I've asked for suggestions for the best ATA at: viewtopic.php?f=14&t=16970&p=66482#p66482 .
Now I'm wondering if I should simply get Internet or SIP/digital phones instead of an ATA since my spouse and I live in a small place and we only really need the phones in one room (my home office). However, I'm not familiar with Internet or SIP phones. Can anyone suggest if this is the better way to go and if so, which Internet or SIP phones would be the best?
I currently have a Nortel Venture analog phone that handles multiple lines. It's easy to conference two lines together and it has a nice speaker and microphone. Is there an Internet or SIP phone that is comparable to this? (This is a nice-to-have, but not a must-have.)
More info on what I'd like to set up:
When I make outgoing calls, I want to show a different number between my business and personal calls. Therefore, I'm thinking of forwarding Number B to Number A and Number D to Number C. Then I'll buy Unlock Keys for Number A and Number C.
For now, I'm not concerned about my fax machine being able to detect the incoming call, because I rarely receive faxes. When I do get a fax, I've just talked to the sender and I will manually get the fax machine to answer the call. But I do want to keep my fax phone number, in case the situation changes in the future. Am I correct to assume that if I want to use my fax machine, I will be forced to use an ATA and ditch the Internet/SIP phone idea?
When people phone for my spouse versus me, we would like to know the difference. With our analog landline, my spouse's phone number is an "Ident-A-Call" number, which means that calls to her number will ring on my personal line, but with a different ring pattern (2 short rings and 1 long ring). Can the Internet/SIP phone be set up so that the when callers call Number D, the Internet/SIP phone will ring with a different ring pattern or ringtone?