T-Mobile’s automatic migration of customers from older mobile plans has mistakenly removed some longstanding free-line promotions, leading to unexpected bill increases and complaints. The error matters because affected customers may lose benefits accumulated through years of loyalty promotions, on top of planned price rises of up to $6 per line.

T-Mobile said technical delays meant discounts were not correctly applied to a small number of accounts, and pledged to restore and backdate them. It is also investigating reports of incorrect Hulu charges and unwanted hotspot-data add-ons, while maintaining that customers should retain their existing promotions and benefits after the migration.