During an active call with the Thuraya SatSleeve, sometimes the “Hold” and “SMS” button are greyed out, why?

Calls that are made within the Thuraya network (e.g. from SatSleeve to SatSleeve or to a Thuraya XT satellite phone) do not support SMS while in an active call and do not support conference calls. However, the Hold and SMS features will be available when one party is a landline or GSM phone.