Not all the system entities are enabled for mobile and tablet phones. We can navigate to Settings | Customization | Customize the system | Components | Entities and can select an entity name to verify if the entity is enabled for mobile phones or not, as shown:

But we can enable this feature for all of our custom entities. Following is a list of the system entities that are enabled for Phone Express and mobile; we can enable or disable these entities using the earlier options: Account, Activity, Appointment, Attachment, Case, Competitor, Connection, Contact, Email, Entitlement, Knowledge Base, Lead, Note, Opportunity, Opportunity Product, Phone Call, Product, Queue, Queue Item, Social Activity, Social Profile, SLA, Task, Team, User, and Web Resource.