I was browsing cityvibe and cross-referencing the phone numbers posted by providers with TER and a few of the numbers had more than one provider.  Does this mean they are part of an agency?

Most likely yes. Does it mention an agency in the TER profile? Many times it won't on Cityvibe.  This is not necessarily a bad thing by the way. A good review is a good review.

That can be really curious sometimes.  Congrats on learning to search by phone number!  A lot of guys are slow to pick up on that.

Check the reviews of the ladies with the same number.  There may or may not be a trend, good or bad.  Are these ads for ladies who claim to be local or visiting?  When a visiting lady has a local number in an ad, it may mean that she is using a third party (perhaps another provider who may be retired) to handle her booking and screening.  The different providers may also just be independent ladies who are either roommates or share a cell phone.

Of course, as Bostonguy says, if the provider you want to see is well reviewed, the question is sort of moot.

Possible reasons that a provider's phone number could show up on multiple TER listings:

1.  Provider has changed her name (can be for many different reasons).

2.  Provider is associated with an agency.

3.  Provider is associated with a group of independent ladies that use a common booking person.

4.  Provider uses a provider friend to book appointments because her english skills may not be very well developed (or she may be shy).

There are probably many other reasons, I'm sure.