Concerning Romans 16:1, the BECNT-Romans commentary (Thomas R. Schreiner, p. 787) has this to say:
Scholars debate, however, whether she held an office. On the one hand, the term diakonos may be generic, denoting various kinds of service and assistance. On the other hand, the term also designates an office (cf. Phil 1:1; 1 Tim. 3:8, 12; see also Ign. Eph. 2.1; Magn. 6.1). Is Paul commending Phoebe because she held the office of deacon, or because she served in a variety of unofficial ways in the church in Cenchreae? It is impossible to be sure, but for several reasons it is likely that she held the office of deacon. First, 1 Tim 3:11 probably identifies women as deacons. Second, the designation "deacon of the church in Cenchreae" suggests that Pheobe served in this special capacity, for this is the only occasion in which the term diakonos is linked with a particular local church. Third, the use of the masculine noun diakonos also suggests that the office is intended. Of course, we need to beware of reading into early church offices the full-fledged development that was realized later. But women deacons were probably appointed early, especially because other women needed assistance from those of their own sex in visitation, baptism, and other matters (cf. Pelagius [de Bruyn 1993: 151]) 4
Footnote 4 reads:
The office of deacon, however, must be distinguished from that of overseer/elder. One should not conclude from Pheobe's role as a deacon that she functioned as a leader of the congregation.
