Question: When is the Bearer Capability 1 Information Element omitted in the mobile radio interface layer 3 specification?
Option 1: When the mobile subscriber has multiple directory numbers for different services.
Option 2: When the MSC wishes to indicate capability for an alternative call mode through fallback.
Option 3: When the bearer capability 2 IE is present.
Option 4: When the mobile subscriber is allocated only one directory number for all services.
Explanation: According to the specification, the Bearer Capability 1 Information Element may be omitted when the mobile subscriber is allocated only one directory number for all services. This is a specific condition that allows for the omission of this information element, highlighting an important detail in the mobile radio interface layer 3 specification.