Section: 5.3.4 Support of Dual Services
Content: The behaviour described in this subclause is used to realize the following required services throughout subclause 5.3.4. The mobile station is not obliged to support the network originated in-call modification procedure. In that case, the mobile station shall, when receiving a MODIFY message, treat the message as unknown and react as described in subclause 8.4. If the mobile station is already prepared to support the procedure in both directions, it shall act as described in this subclause. Alternate Speech/Group 3 fax (Teleservice 61 according to 3GPP TS 22.003[ Circuit Teleservices supported by a Public Land Mobile Network (PLMN) ]  [4]).