A Jefferies analyst downgraded shares of Acme Packet Inc., a Bedford-based telecommunications company, to ‘‘hold’’ from “buy.’’ Analyst James Kisner also set a 12-month price target of $21 per share. Jefferies cited a change in spending by US carriers to focus on expanding their wireless business, Reuters reported. Acme Packet has more than 1,600 customers in more than 107 countries.