As long as the phone network can reach the 6250, then a SIP phone can register to it. The fact that the traffic traverses a router along the way isn't itself a problem.
Gotchas of making this work include NAT between the phone and the 6250 by an external device (try to avoid this if at all possible), configuration and provisioning of the phone itself. If the SIP phone will also connect to another VoIP system, then things get more complicated, including (this isn't an exhaustive list...)
- Is this the only device to which the phone will register?
- If not, do you want a separate line button on the phone for it?
- How is the SIP phone going to be configured (tftp/ftp/http/https server, etc.)
- Are the dialplans compatible with the other system?
- 911 concerns if the phone is remote.