A professional is not required to repair an AT&T phone line inside of a building, because the wiring is owned by the building owner. Phone lines that connect the building to AT&T's main network belong to AT&T and must be repaired by professionals from AT&T.
In a multiunit dwelling such as an apartment building, the indoor phone wiring belongs to the building owner unless otherwise specified in the lease. Tenants must defer to the landlord's policy on repairs; landlords are within their rights to require professional repair on wiring that they own.
AT&T has sole responsibility for the wiring it owns but no responsibility for customer-owned wiring inside buildings. The company may charge for work on customer-owned phone lines or may decline to do the work at all. In these cases, DIY repair and third-party professional electricians are options.