The best thing you can do is home run a CAT-5E solid conductor cable back to the DMARC. (Outside box where the PSTN copper line comes in.) You can put one DSL filter here in parallel with the CAT-5E for the rest of the RJ-11 voice outlets instead of putting one on each phone outlet.
Why you don't want to use old house wiring or
Straight pair, cheap voice cables (usually found installed in pre-1990's buildings) suffered from noise, emi pickup, and crosstalk issues.
The old wiring blocks don't even meet cat-3 specifications. (Screw and terminal) they were replaced with the 66-block style that do meet the requirements, you can find mini versions at Lowe's
or home depot etc.
- CAT-3: Should be used for 10Mbps or less.
- CAT5: Should be used up to 100Mbps.
- CAT-5E: Should be used for 100Mbps ~ 1000Mbps. (If you are lucky enough to be offered these speeds by the provider)
"are there any types and quality standards for RJ11 cables, such as Ethernet cat 5, cat 6 and?"
~Yes, working at a local CLEC here in PA, CAT-5E was mandatory for new installations. This was a recommendation from Verizon, because we were using their network equipment etc. (They told us mismatching awg/category types could cause signal reflection in the inside wiring.)
To be honest, it doesn't even make sense to use the other cables categories, at this point cat-5E is cheap enough that it should be installed if you are anticipating higher speeds in the future offered by the ISP, or if you just want to ensure the best possible SNR ratio from DMARC to modem.
Sometimes this isn't an option, for instance if you are renting an apartment. The best thing to do in that scenario is to see if the landlord would let you hire someone to run the cable for you.
If all else fails, you could try the extension. But if you don't pop open a RJ-11 outlet and take a peek at the wiring to see what type it is, you could actually degrade the signal with an impedance mis-match, I.e, the AWG sizes are different...