While there is an ISO standard for pet chips, there are several competing versions that are also in use. I think my vet said there were 3 or 4 different kinds and each had a different kind of reader.
There's three different frequencies commonly used in microchips for pets: 125, 128, and 134.2 KHz (the last being the ISO standard). Hope your local animal shelter has a universal chip reader, because I doubt your vet knows whether the chips they use are standards-compliant or not.