|Those are two different subnets. Does not appear the issue is vlans but of subnetting. And the provider is right given this circumstance.|
Think about this a second. In this situation the phone is a hub. It is also an ip device. So ip has to flow thru the phone to get to the pc.
Any more vlans than the default one can't work here unless these phones also support vlan tagging like you get with a managed switch. Having two vlans on the same wire [to pc not trunking] is pointless imo. Point of a vlan is to isolate traffic. One wire to phone and pc defeats this.
Normally Voip phones are on their own wiring diagram not shared with the data network. This can then support a vlan for phone and a vlan for pcs. Additionally this situation supports two subnets. There is no communication between pc and phone.
This is not your case.
There is no reason I can see to have two subnets in this circumstance. The phone folks are correct in that with two subnets without a router [at each phone] the pc app can't talk to the phone.