VoIp proposed to replace T1 iSeries uses

Customer had salesperson in today offering to replace all T1s with their 
managed voip system.
Currently one T1 for phone system, one T1 for data (lan access to web, iSeries 
webserver
and ftp up & down, remote access, etc). Also current T1 provider is our managed 
firewall provider.

As we go thru this analysis - what should I watch out for? 
When they talked about mixing voice & data, and voice taking priority, I get 
nervous.
Also, this proposal is a "managed" service, from their location. 
And finally, salesperson's claim their internet phones, with no configuration, 
can ethernet plug into 
any network in the world and autocfg   (cat5 into pc plugs into phone, then 
another cat5 to pc).
I would think a lot of router setups, home dsl or broadband, or firewalls might 
block this.

My responsibility is keeping iSeries and all services secure & always running.

jim franz