Unfortunately we can't recommend soft phones and i certainly don't know of anything that can handle that many accounts configured to it.
Is there any chance you could email to firstname.lastname@example.org
and give us more of a detailed idea, of what is you need and how you want it to work?
Voipfone Customer Services
iNet Telecoms Ltd (Voipfone)
227 Marsh Wall
Registered number: 05168033
Vat Registration Number 858850966
Telephone: 020 7043 5555
Fax: 020 7043 5556