Many people have posted how you may use an in-line Micro-Filter in lieu of a POTS Splitter to filter the whole house; saving the cost of purchasing a POTS Splitter.  You may, but you are quite likely asking for trouble.  The inline Micro-Filter is, as you can see in the photo, is a single stage filter consisting of an inductive device and a capacitance device.  The circuit board is bare.  The POTS Splitter is a three-stage filter and much more capable of separating analog voice/fax/dial-up modem from the DSL modem.  The circuit board has a clear coating to protect it from the moisture that will be present in an outdoor location.  The POTS Splitter is designed for filtering the whole house, and for use in an outdoor location.

If you are too close to the Central Office, or the Remote Terminal if you are serviced from such, the strength of the DSL signal can overwhelm the lesser in-line Micro-Filter and allow noise to affect analog devices.  If you are too far from the Central Office, or the Remote Terminal if you are serviced from such, the extra wires, jacks, and filters will cause a loading of your DSL line that will lower the maximum download speed.  If this goes lowers than your capped speed you will experience lesser performance.