Voice VLAN allows you to specify what the VLANs are, so you should be able to use VLAN 1, but if you are using VLAN 1 and 100 you probably could just create VLAN 100 and not bother with the voice vlan stuff. What port mode is the port that you are trying to set to tagged in? Access mode can only do untagged. If it was general mode you could set it to tagged of 100 and a PVID of 1 which would put untagged traffic on vlan 1. 
We've added our first Dell PowerConnect 8024 switch to an existing network consisting of HP 2910 switches.  We have only 2 VLANS configured:  the default (1) and voice (100).  On the HP switches all ports in the default VLAN are set to "untagged" and all ports in the voice VLAN are set to "tagged". I need to configure the PC8024 to coexist with the existing network so that the Mitel phones can communicate.  We are not familiar with the PC8024 switch, are evaluating it as a potential solution for upgrading our backbone and DR subnet from 1Gb to 10Gb.  For the backbone we need the voice traffic to be handled properly, so we need to configure the 8024 to work with the existing voice VLAN. What VLAN configuration settings do we need on the PC8024 to coexist with the existing network configuration for the voice (100) VLAN on the HP switches?
Interface PC8024 with existing voice VLAN (100) on HP 2910 switches