Chelsea striker Fran Kirby and Lyon defender Alex Greenwood have withdrawn from the England squad with injury.
The Lionesses face Belgium on Thursday, followed by Norway next Tuesday in friendly matches.
Coach Phil Neville had initially named a 24-strong squad, which included 19 players who featured in the Women’s World Cup, as well as handing call-ups to four debutants.
Midfielder Williams, 35, has more caps than any other England player, male or female, with 168 appearances but was not selected for this summer’s World Cup.
Chelsea’s Bethany England, 25, and Manchester City’s Aoife Mannion, 23, have been handed their first senior call-ups, alongside two of the Young Lionesses’ bronze medal-winning Under-20 World Cup squad, 21-year-old Sandy MacIver and 20-year-old Anna Patten.
England, who reached the semi-finals of the World Cup in France – losing to eventual winners the United States – face Belgium in Leuven before travelling to Bergen to take on their World Cup quarter-final opponents Norway.
Goalkeepers: Mary Earps, Sandy MacIver, Ellie Roebuck, Carly Telford
Midfielders: Lucy Bronze, Jade Moore, Lucy Staniforth, Georgia Stanway, Keira Walsh, Fara Williams
Forwards: Toni Duggan, Bethany England, Beth Mead, Nikita Parris, Jodie Taylor