Is it possible to call a telephone number with an extension?

Not 100% sure if this is the answer, but from the RFC you linked on page 14:

tel:+358-555-1234567;postd=pp22

The above URL instructs the local entity to place a voice call to +358-555-1234567, then wait for an implementation-dependent time (for example, two seconds) and emit two DTMF dialing tones "2" on the line (for example, to choose a particular extension number, or to invoke a particular service).

Hope this helps.

If it is a phone system where you dial the main number, hear a prompt and then enter the extension number then you can include a pause in your number to allow for the call to be picked up. Per the RFC 2806 that you linked to this is done using a "p" pause character for each 1 second pause.