Generally speaking invalid device name means that the line device pass is not valid from the perspective of being enumerated when dialogic system services were started. In the case of IP you need to make sure that IP_VIRTBOARD structure was correctly populated with number of maxcalls in this case. Such that IPT device enumeration would be correct.
Refer to chapter 2.3 of the GC IP TUG:
www.dialogic.com/.../globalcall_for_ip_hmp_v12.pdf
Jeff