Problem: Switchports will not go into voice VLAN no matter what you do: -ISL

Problem:  Switchports will not go into voice VLAN no matter what you do: Very old switches: IOS ™ C2900XL Software (C2900XL-C3H2S-M) . Eg 2924. To enable phones to work, need to trunk the switchports, and because they are so old, they are encapsulating in ISL. Need to add this code for each port:switchport trunk encapsulation dot1q switchport mode trunk switchport voice vlan X