The immense size and liquidity of the agency MBS market may be attracting a new group of investors bent on making interest-rate plays by buying large positions in securities duration, according to analysts at Bear Stearns. “MBS may be the only market where a portfolio can make one phone call and get a $35 billion rates offering,” said Steven Abrahams, a Bear Stearns analyst, in a recent report. “The availability of that kind of size…